diff --git a/AGENTS.md b/AGENTS.md
index 736e1bea1d4d..df17b9484308 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -368,9 +368,6 @@ An inline `@phan-suppress-next-line <Rule> -- <reason>` is acceptable ONLY as a
 - The "phan: Update wpcom stubs" PR is machine-generated and gets rebased/recreated on every job run — never hand-edit it, your changes will be overwritten.
 - `.phan/stubs/wpcom-stubs.php` is likewise generated (its header says so). Never edit it directly to add a symbol — add it to `stub-defs.php` in wpcom instead.
 
-## Maintaining this file
+## Maintaining This File
 
-Keep this file for knowledge useful to almost every future agent session in this project.
-Do not repeat what the codebase already shows; point to the authoritative file or command instead.
-Prefer rewriting or pruning existing entries over appending new ones.
-When updating this file, preserve this bar for all agents and keep entries concise.
+If you discover a pattern or pitfall not covered here, mention it to the developer so they can decide whether to update this file.
diff --git a/CLAUDE.md b/CLAUDE.md
deleted file mode 100644
index a9d4d2694af2..000000000000
--- a/CLAUDE.md
+++ /dev/null
@@ -1,2 +0,0 @@
-<!-- Points Claude at AGENTS.md via import; edit AGENTS.md, not this file. -->
-@AGENTS.md
diff --git a/projects/packages/premium-analytics/changelog/change-wooa7s-2100-daterange-zoned b/projects/packages/premium-analytics/changelog/change-wooa7s-2100-daterange-zoned
new file mode 100644
index 000000000000..19b1c964a989
--- /dev/null
+++ b/projects/packages/premium-analytics/changelog/change-wooa7s-2100-daterange-zoned
@@ -0,0 +1,4 @@
+Significance: patch
+Type: changed
+
+Date range: Type the range bounds as zoned instants, so day boundaries are measured on the site's clock. A range whose end precedes its start now reports no period length instead of a one-hour window.
diff --git a/projects/packages/premium-analytics/packages/datetime/README.md b/projects/packages/premium-analytics/packages/datetime/README.md
index 0df9f7b24a91..8ee42d98d482 100644
--- a/projects/packages/premium-analytics/packages/datetime/README.md
+++ b/projects/packages/premium-analytics/packages/datetime/README.md
@@ -141,8 +141,8 @@ Calculates comparison date ranges based on predefined presets.
 
 ```typescript
 const reference = {
-	from: new Date( '2024-01-15' ),
-	to: new Date( '2024-01-21' ),
+	from: localTZDate( '2024-01-15', 'America/New_York' ),
+	to: localTZDate( '2024-01-21', 'America/New_York' ),
 };
 const comparison = getComparisonRangeFromPreset( reference, 'previous-period' );
 // Returns dates for Jan 8-14, 2024
@@ -221,11 +221,14 @@ twelve-month window as 12 months.
 
 ```typescript
 type DateRange = {
-	from?: Date;
-	to?: Date;
+	from?: TZDate;
+	to?: TZDate;
 };
 ```
 
+Both bounds stay optional: `resolveBucketStamp` returns `undefined` for a bound
+it cannot resolve, and the chart passes that straight through.
+
 ### `ComparisonPresetId`
 
 ```typescript
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/comparison-presets.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/comparison-presets.test.ts
index 6fedf0120a63..079fc0e3a6f1 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/comparison-presets.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/comparison-presets.test.ts
@@ -3,6 +3,37 @@
  */
 import { COMPARISON_PRESETS, isComparisonPresetId } from '../get-comparison-range';
 import { getComparisonOptions } from '../presets';
+import { createTZDateFromParts } from '../tz';
+import type { TZDate } from '@date-fns/tz';
+/**
+ * A site timezone with a fixed offset, so every expectation below holds
+ * whatever timezone the machine running the suite is in.
+ */
+const SITE_ZONE = 'Asia/Taipei';
+
+/**
+ * Build a site-local date from the parts `new Date()` takes.
+ *
+ * @param year    - Full year.
+ * @param month   - 0-indexed month.
+ * @param day     - Day of month.
+ * @param hours   - Hour of day.
+ * @param minutes - Minute of hour.
+ * @param seconds - Second of minute.
+ * @param ms      - Millisecond of second.
+ * @return The date.
+ */
+function siteDate(
+	year: number,
+	month: number,
+	day: number,
+	hours = 0,
+	minutes = 0,
+	seconds = 0,
+	ms = 0
+): TZDate {
+	return createTZDateFromParts( [ year, month, day, hours, minutes, seconds, ms ], SITE_ZONE );
+}
 
 /**
  * A day-aligned range, inclusive on both ends. Months are 0-based.
@@ -10,8 +41,8 @@ import { getComparisonOptions } from '../presets';
  * @param to
  */
 const daysRange = ( from: [ number, number, number ], to: [ number, number, number ] ) => ( {
-	from: new Date( from[ 0 ], from[ 1 ], from[ 2 ], 0, 0, 0, 0 ),
-	to: new Date( to[ 0 ], to[ 1 ], to[ 2 ], 23, 59, 59, 999 ),
+	from: siteDate( from[ 0 ], from[ 1 ], from[ 2 ], 0, 0, 0, 0 ),
+	to: siteDate( to[ 0 ], to[ 1 ], to[ 2 ], 23, 59, 59, 999 ),
 } );
 
 const ids = ( range: Parameters< typeof getComparisonOptions >[ 0 ] ) =>
@@ -42,7 +73,7 @@ describe( 'comparison options', () => {
 
 	it( 'returns nothing for an incomplete or inverted range', () => {
 		expect( getComparisonOptions( {} ) ).toEqual( [] );
-		expect( getComparisonOptions( { from: new Date( 2026, 7, 30 ) } ) ).toEqual( [] );
+		expect( getComparisonOptions( { from: siteDate( 2026, 7, 30 ) } ) ).toEqual( [] );
 		expect( getComparisonOptions( daysRange( [ 2026, 7, 30 ], [ 2026, 7, 29 ] ) ) ).toEqual( [] );
 	} );
 
@@ -65,8 +96,8 @@ describe( 'comparison options', () => {
 
 	it( 'reads a rolling 24-hour window in hours', () => {
 		const last24Hours = {
-			from: new Date( 2026, 7, 30, 15, 0, 0, 0 ),
-			to: new Date( 2026, 7, 31, 14, 59, 59, 999 ),
+			from: siteDate( 2026, 7, 30, 15, 0, 0, 0 ),
+			to: siteDate( 2026, 7, 31, 14, 59, 59, 999 ),
 		};
 
 		const options = getComparisonOptions( last24Hours );
@@ -79,8 +110,8 @@ describe( 'comparison options', () => {
 		] );
 		expect( options[ 0 ].label ).toBe( 'Previous 24 hours' );
 		expect( options[ 1 ].range ).toEqual( {
-			from: new Date( 2026, 7, 23, 15, 0, 0, 0 ),
-			to: new Date( 2026, 7, 24, 14, 59, 59, 999 ),
+			from: siteDate( 2026, 7, 23, 15, 0, 0, 0 ),
+			to: siteDate( 2026, 7, 24, 14, 59, 59, 999 ),
 		} );
 	} );
 
@@ -199,16 +230,16 @@ describe( 'comparison options', () => {
 
 	it( 'reads a drilled single hour as the previous hour', () => {
 		const hour = {
-			from: new Date( 2026, 7, 31, 14, 0, 0, 0 ),
-			to: new Date( 2026, 7, 31, 14, 59, 59, 999 ),
+			from: siteDate( 2026, 7, 31, 14, 0, 0, 0 ),
+			to: siteDate( 2026, 7, 31, 14, 59, 59, 999 ),
 		};
 
 		const options = getComparisonOptions( hour );
 
 		expect( options[ 0 ].label ).toBe( 'Previous hour' );
 		expect( options[ 0 ].range ).toEqual( {
-			from: new Date( 2026, 7, 31, 13, 0, 0, 0 ),
-			to: new Date( 2026, 7, 31, 13, 59, 59, 999 ),
+			from: siteDate( 2026, 7, 31, 13, 0, 0, 0 ),
+			to: siteDate( 2026, 7, 31, 13, 59, 59, 999 ),
 		} );
 	} );
 
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date-range-span.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date-range-span.test.ts
index 77bb76ae6686..b845d2618150 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date-range-span.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date-range-span.test.ts
@@ -2,11 +2,17 @@
  * Internal dependencies
  */
 import { getDateRangeSpan } from '../date-range-span';
+import { createTZDateFromParts } from '../tz';
+import type { TZDate } from '@date-fns/tz';
 
 /**
- * Build a local-time date. Dates are constructed from parts rather than parsed
- * from ISO strings so the day boundaries land in the machine's timezone, which
- * is the frame `date-fns` reads.
+ * A site timezone with a fixed offset, so every expectation below holds
+ * whatever timezone the machine running the suite is in.
+ */
+const SITE_ZONE = 'Asia/Taipei';
+
+/**
+ * Build a site-local date.
  *
  * @param year    - Full year.
  * @param month   - 1-indexed month.
@@ -15,8 +21,8 @@ import { getDateRangeSpan } from '../date-range-span';
  * @param minutes - Minute of hour.
  * @return The date.
  */
-function at( year: number, month: number, day: number, hours = 0, minutes = 0 ): Date {
-	return new Date( year, month - 1, day, hours, minutes, 0, 0 );
+function at( year: number, month: number, day: number, hours = 0, minutes = 0 ): TZDate {
+	return createTZDateFromParts( [ year, month - 1, day, hours, minutes, 0, 0 ], SITE_ZONE );
 }
 
 /**
@@ -27,8 +33,8 @@ function at( year: number, month: number, day: number, hours = 0, minutes = 0 ):
  * @param day   - Day of month.
  * @return The end of that day.
  */
-function endOf( year: number, month: number, day: number ): Date {
-	return new Date( year, month - 1, day, 23, 59, 59, 999 );
+function endOf( year: number, month: number, day: number ): TZDate {
+	return createTZDateFromParts( [ year, month - 1, day, 23, 59, 59, 999 ], SITE_ZONE );
 }
 
 describe( 'getDateRangeSpan', () => {
@@ -38,6 +44,10 @@ describe( 'getDateRangeSpan', () => {
 		expect( getDateRangeSpan( { from: at( 2026, 7, 21 ) } ) ).toBeNull();
 	} );
 
+	it( 'returns null when the range runs backwards', () => {
+		expect( getDateRangeSpan( { from: endOf( 2026, 7, 28 ), to: at( 2026, 6, 29 ) } ) ).toBeNull();
+	} );
+
 	it( 'counts whole days inclusively', () => {
 		expect( getDateRangeSpan( { from: at( 2026, 7, 21 ), to: endOf( 2026, 7, 27 ) } ) ).toEqual( {
 			unit: 'day',
@@ -61,7 +71,7 @@ describe( 'getDateRangeSpan', () => {
 		expect(
 			getDateRangeSpan( {
 				from: at( 2026, 7, 28, 19 ),
-				to: new Date( 2026, 6, 29, 18, 59, 59, 999 ),
+				to: createTZDateFromParts( [ 2026, 6, 29, 18, 59, 59, 999 ], SITE_ZONE ),
 			} )
 		).toEqual( { unit: 'hour', value: 24 } );
 	} );
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/get-comparison-range.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/get-comparison-range.test.ts
index 08b0bfe1518b..09d47bc31fb6 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/get-comparison-range.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/get-comparison-range.test.ts
@@ -8,32 +8,62 @@ import { differenceInDays } from 'date-fns';
 import { getDateRangeSpan } from '../date-range-span';
 import { COMPARISON_PRESETS, getComparisonRangeFromPreset } from '../get-comparison-range';
 import { createTZDateFromParts } from '../tz';
+import type { TZDate } from '@date-fns/tz';
+/**
+ * A site timezone with a fixed offset, so every expectation below holds
+ * whatever timezone the machine running the suite is in.
+ */
+const SITE_ZONE = 'Asia/Taipei';
+
+/**
+ * Build a site-local date from the parts `new Date()` takes.
+ *
+ * @param year    - Full year.
+ * @param month   - 0-indexed month.
+ * @param day     - Day of month.
+ * @param hours   - Hour of day.
+ * @param minutes - Minute of hour.
+ * @param seconds - Second of minute.
+ * @param ms      - Millisecond of second.
+ * @return The date.
+ */
+function siteDate(
+	year: number,
+	month: number,
+	day: number,
+	hours = 0,
+	minutes = 0,
+	seconds = 0,
+	ms = 0
+): TZDate {
+	return createTZDateFromParts( [ year, month, day, hours, minutes, seconds, ms ], SITE_ZONE );
+}
 
 describe( 'getComparisonRangeFromPreset', () => {
 	it( 'returns undefined when the reference range is incomplete', () => {
 		expect(
-			getComparisonRangeFromPreset( { from: new Date( 2026, 6, 1 ) }, 'previous-period' )
+			getComparisonRangeFromPreset( { from: siteDate( 2026, 6, 1 ) }, 'previous-period' )
 		).toBeUndefined();
 		expect( getComparisonRangeFromPreset( {}, 'previous-period' ) ).toBeUndefined();
 	} );
 
 	describe( 'day-aligned references', () => {
 		const reference = {
-			from: new Date( 2026, 5, 1, 0, 0, 0, 0 ),
-			to: new Date( 2026, 5, 7, 23, 59, 59, 999 ),
+			from: siteDate( 2026, 5, 1, 0, 0, 0, 0 ),
+			to: siteDate( 2026, 5, 7, 23, 59, 59, 999 ),
 		};
 
 		it( 'mirrors the previous period on day bounds', () => {
 			expect( getComparisonRangeFromPreset( reference, 'previous-period' ) ).toEqual( {
-				from: new Date( 2026, 4, 25, 0, 0, 0, 0 ),
-				to: new Date( 2026, 4, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 4, 25, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 4, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 
 		it( 'shifts the previous month, clamping to day bounds', () => {
 			expect( getComparisonRangeFromPreset( reference, 'previous-month' ) ).toEqual( {
-				from: new Date( 2026, 4, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 4, 7, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 4, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 4, 7, 23, 59, 59, 999 ),
 			} );
 		} );
 	} );
@@ -42,14 +72,14 @@ describe( 'getComparisonRangeFromPreset', () => {
 		// A rolling 24-hour window ending mid-afternoon, ends inclusive as the
 		// presets build them (`endOfHour`).
 		const reference = {
-			from: new Date( 2026, 6, 9, 14, 30, 0, 0 ),
-			to: new Date( 2026, 6, 10, 14, 29, 59, 999 ),
+			from: siteDate( 2026, 6, 9, 14, 30, 0, 0 ),
+			to: siteDate( 2026, 6, 10, 14, 29, 59, 999 ),
 		};
 
 		it( 'mirrors the exact previous window for previous-period', () => {
 			expect( getComparisonRangeFromPreset( reference, 'previous-period' ) ).toEqual( {
-				from: new Date( 2026, 6, 8, 14, 30, 0, 0 ),
-				to: new Date( 2026, 6, 9, 14, 29, 59, 999 ),
+				from: siteDate( 2026, 6, 8, 14, 30, 0, 0 ),
+				to: siteDate( 2026, 6, 9, 14, 29, 59, 999 ),
 			} );
 		} );
 
@@ -63,27 +93,27 @@ describe( 'getComparisonRangeFromPreset', () => {
 			// The `last-24-hours` shape: shifting by the exclusive span landed `to` on
 			// the reference's own `from`, pulling hourly buckets one hour late.
 			const last24Hours = {
-				from: new Date( 2026, 7, 17, 15, 0, 0, 0 ),
-				to: new Date( 2026, 7, 18, 14, 59, 59, 999 ),
+				from: siteDate( 2026, 7, 17, 15, 0, 0, 0 ),
+				to: siteDate( 2026, 7, 18, 14, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( last24Hours, 'previous-period' ) ).toEqual( {
-				from: new Date( 2026, 7, 16, 15, 0, 0, 0 ),
-				to: new Date( 2026, 7, 17, 14, 59, 59, 999 ),
+				from: siteDate( 2026, 7, 16, 15, 0, 0, 0 ),
+				to: siteDate( 2026, 7, 17, 14, 59, 59, 999 ),
 			} );
 		} );
 
 		it( 'keeps the time of day for previous-month', () => {
 			expect( getComparisonRangeFromPreset( reference, 'previous-month' ) ).toEqual( {
-				from: new Date( 2026, 5, 9, 14, 30, 0, 0 ),
-				to: new Date( 2026, 5, 10, 14, 29, 59, 999 ),
+				from: siteDate( 2026, 5, 9, 14, 30, 0, 0 ),
+				to: siteDate( 2026, 5, 10, 14, 29, 59, 999 ),
 			} );
 		} );
 
 		it( 'keeps the time of day for previous-year', () => {
 			expect( getComparisonRangeFromPreset( reference, 'previous-year' ) ).toEqual( {
-				from: new Date( 2025, 6, 9, 14, 30, 0, 0 ),
-				to: new Date( 2025, 6, 10, 14, 29, 59, 999 ),
+				from: siteDate( 2025, 6, 9, 14, 30, 0, 0 ),
+				to: siteDate( 2025, 6, 10, 14, 29, 59, 999 ),
 			} );
 		} );
 	} );
@@ -92,8 +122,8 @@ describe( 'getComparisonRangeFromPreset', () => {
 		// A rolling 24-hour window at the end of March; February is shorter,
 		// so a plain calendar shift would collapse both endpoints onto Feb 28.
 		const endOfMarch = {
-			from: new Date( 2026, 2, 30, 14, 0, 0, 0 ),
-			to: new Date( 2026, 2, 31, 14, 0, 0, 0 ),
+			from: siteDate( 2026, 2, 30, 14, 0, 0, 0 ),
+			to: siteDate( 2026, 2, 31, 14, 0, 0, 0 ),
 		};
 
 		it.each( COMPARISON_PRESETS )( 'preserves the window duration for %s', presetId => {
@@ -105,32 +135,32 @@ describe( 'getComparisonRangeFromPreset', () => {
 
 		it( 'keeps a 24h window for previous-month when both endpoints would clamp', () => {
 			expect( getComparisonRangeFromPreset( endOfMarch, 'previous-month' ) ).toEqual( {
-				from: new Date( 2026, 1, 27, 14, 0, 0, 0 ),
-				to: new Date( 2026, 1, 28, 14, 0, 0, 0 ),
+				from: siteDate( 2026, 1, 27, 14, 0, 0, 0 ),
+				to: siteDate( 2026, 1, 28, 14, 0, 0, 0 ),
 			} );
 		} );
 
 		it( 'keeps a 48h window for previous-month when one endpoint would clamp', () => {
 			const rolling48h = {
-				from: new Date( 2026, 2, 30, 14, 0, 0, 0 ),
-				to: new Date( 2026, 3, 1, 14, 0, 0, 0 ),
+				from: siteDate( 2026, 2, 30, 14, 0, 0, 0 ),
+				to: siteDate( 2026, 3, 1, 14, 0, 0, 0 ),
 			};
 
 			expect( getComparisonRangeFromPreset( rolling48h, 'previous-month' ) ).toEqual( {
-				from: new Date( 2026, 1, 27, 14, 0, 0, 0 ),
-				to: new Date( 2026, 2, 1, 14, 0, 0, 0 ),
+				from: siteDate( 2026, 1, 27, 14, 0, 0, 0 ),
+				to: siteDate( 2026, 2, 1, 14, 0, 0, 0 ),
 			} );
 		} );
 
 		it( 'keeps a 24h window for previous-year across leap day', () => {
 			const leapDay = {
-				from: new Date( 2028, 1, 28, 14, 0, 0, 0 ),
-				to: new Date( 2028, 1, 29, 14, 0, 0, 0 ),
+				from: siteDate( 2028, 1, 28, 14, 0, 0, 0 ),
+				to: siteDate( 2028, 1, 29, 14, 0, 0, 0 ),
 			};
 
 			expect( getComparisonRangeFromPreset( leapDay, 'previous-year' ) ).toEqual( {
-				from: new Date( 2027, 1, 27, 14, 0, 0, 0 ),
-				to: new Date( 2027, 1, 28, 14, 0, 0, 0 ),
+				from: siteDate( 2027, 1, 27, 14, 0, 0, 0 ),
+				to: siteDate( 2027, 1, 28, 14, 0, 0, 0 ),
 			} );
 		} );
 	} );
@@ -139,38 +169,38 @@ describe( 'getComparisonRangeFromPreset', () => {
 		it.each( [
 			[
 				'a rolling 30-day window',
-				new Date( 2026, 6, 21, 0, 0, 0, 0 ),
-				new Date( 2026, 7, 19, 23, 59, 59, 999 ),
-				new Date( 2026, 5, 20, 0, 0, 0, 0 ),
-				new Date( 2026, 6, 19, 23, 59, 59, 999 ),
+				siteDate( 2026, 6, 21, 0, 0, 0, 0 ),
+				siteDate( 2026, 7, 19, 23, 59, 59, 999 ),
+				siteDate( 2026, 5, 20, 0, 0, 0, 0 ),
+				siteDate( 2026, 6, 19, 23, 59, 59, 999 ),
 			],
 			[
 				'a window whose end clamps in a shorter month',
-				new Date( 2026, 2, 2, 0, 0, 0, 0 ),
-				new Date( 2026, 2, 31, 23, 59, 59, 999 ),
-				new Date( 2026, 0, 30, 0, 0, 0, 0 ),
-				new Date( 2026, 1, 28, 23, 59, 59, 999 ),
+				siteDate( 2026, 2, 2, 0, 0, 0, 0 ),
+				siteDate( 2026, 2, 31, 23, 59, 59, 999 ),
+				siteDate( 2026, 0, 30, 0, 0, 0, 0 ),
+				siteDate( 2026, 1, 28, 23, 59, 59, 999 ),
 			],
 			[
 				'a week spanning a year boundary',
-				new Date( 2025, 11, 29, 0, 0, 0, 0 ),
-				new Date( 2026, 0, 4, 23, 59, 59, 999 ),
-				new Date( 2025, 10, 28, 0, 0, 0, 0 ),
-				new Date( 2025, 11, 4, 23, 59, 59, 999 ),
+				siteDate( 2025, 11, 29, 0, 0, 0, 0 ),
+				siteDate( 2026, 0, 4, 23, 59, 59, 999 ),
+				siteDate( 2025, 10, 28, 0, 0, 0, 0 ),
+				siteDate( 2025, 11, 4, 23, 59, 59, 999 ),
 			],
 			[
 				'a window that starts on the 1st but stops short of the month end',
-				new Date( 2026, 2, 1, 0, 0, 0, 0 ),
-				new Date( 2026, 2, 15, 23, 59, 59, 999 ),
-				new Date( 2026, 1, 1, 0, 0, 0, 0 ),
-				new Date( 2026, 1, 15, 23, 59, 59, 999 ),
+				siteDate( 2026, 2, 1, 0, 0, 0, 0 ),
+				siteDate( 2026, 2, 15, 23, 59, 59, 999 ),
+				siteDate( 2026, 1, 1, 0, 0, 0, 0 ),
+				siteDate( 2026, 1, 15, 23, 59, 59, 999 ),
 			],
 			[
 				'a window whose start has no counterpart a month back',
-				new Date( 2026, 0, 31, 0, 0, 0, 0 ),
-				new Date( 2026, 2, 1, 23, 59, 59, 999 ),
-				new Date( 2026, 0, 3, 0, 0, 0, 0 ),
-				new Date( 2026, 1, 1, 23, 59, 59, 999 ),
+				siteDate( 2026, 0, 31, 0, 0, 0, 0 ),
+				siteDate( 2026, 2, 1, 23, 59, 59, 999 ),
+				siteDate( 2026, 0, 3, 0, 0, 0, 0 ),
+				siteDate( 2026, 1, 1, 23, 59, 59, 999 ),
 			],
 		] )(
 			'keeps the reference length for previous-month with %s',
@@ -184,13 +214,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 
 		it( 'keeps the reference length for previous-year across a leap day', () => {
 			const reference = {
-				from: new Date( 2028, 1, 20, 0, 0, 0, 0 ),
-				to: new Date( 2028, 2, 5, 23, 59, 59, 999 ),
+				from: siteDate( 2028, 1, 20, 0, 0, 0, 0 ),
+				to: siteDate( 2028, 2, 5, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( reference, 'previous-year' ) ).toEqual( {
-				from: new Date( 2027, 1, 19, 0, 0, 0, 0 ),
-				to: new Date( 2027, 2, 5, 23, 59, 59, 999 ),
+				from: siteDate( 2027, 1, 19, 0, 0, 0, 0 ),
+				to: siteDate( 2027, 2, 5, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -198,8 +228,8 @@ describe( 'getComparisonRangeFromPreset', () => {
 			'covers the same number of days as the reference for %s',
 			presetId => {
 				const reference = {
-					from: new Date( 2026, 0, 31, 0, 0, 0, 0 ),
-					to: new Date( 2026, 2, 1, 23, 59, 59, 999 ),
+					from: siteDate( 2026, 0, 31, 0, 0, 0, 0 ),
+					to: siteDate( 2026, 2, 1, 23, 59, 59, 999 ),
 				};
 				const comparison = getComparisonRangeFromPreset( reference, presetId );
 
@@ -211,37 +241,37 @@ describe( 'getComparisonRangeFromPreset', () => {
 	describe( 'whole calendar months', () => {
 		it( 'sets a whole month against the whole month before it', () => {
 			const march = {
-				from: new Date( 2026, 2, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 2, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 2, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 2, 31, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( march, 'previous-month' ) ).toEqual( {
-				from: new Date( 2026, 1, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 1, 28, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 1, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 1, 28, 23, 59, 59, 999 ),
 			} );
 		} );
 
 		it( 'keeps a multi-month window on month bounds', () => {
 			const janToFeb = {
-				from: new Date( 2026, 0, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 1, 28, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 0, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 1, 28, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( janToFeb, 'previous-month' ) ).toEqual( {
-				from: new Date( 2025, 11, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 0, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 11, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 0, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 
 		it( 'sets a leap February against the shorter one a year back', () => {
 			const february2028 = {
-				from: new Date( 2028, 1, 1, 0, 0, 0, 0 ),
-				to: new Date( 2028, 1, 29, 23, 59, 59, 999 ),
+				from: siteDate( 2028, 1, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2028, 1, 29, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( february2028, 'previous-year' ) ).toEqual( {
-				from: new Date( 2027, 1, 1, 0, 0, 0, 0 ),
-				to: new Date( 2027, 1, 28, 23, 59, 59, 999 ),
+				from: siteDate( 2027, 1, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2027, 1, 28, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -267,13 +297,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 		 */
 		it( 'sets a whole month against the whole month before it for previous-period', () => {
 			const march = {
-				from: new Date( 2026, 2, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 2, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 2, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 2, 31, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( march, 'previous-period' ) ).toEqual( {
-				from: new Date( 2026, 1, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 1, 28, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 1, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 1, 28, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -283,13 +313,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 		 */
 		it( 'sets a calendar year against the previous calendar year for previous-period', () => {
 			const year2025 = {
-				from: new Date( 2025, 0, 1, 0, 0, 0, 0 ),
-				to: new Date( 2025, 11, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 0, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 11, 31, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( year2025, 'previous-period' ) ).toEqual( {
-				from: new Date( 2024, 0, 1, 0, 0, 0, 0 ),
-				to: new Date( 2024, 11, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 0, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2024, 11, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -300,13 +330,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 		 */
 		it( 'steps a rolling 12-month window back by its month count for previous-period', () => {
 			const last12Months = {
-				from: new Date( 2025, 7, 31, 0, 0, 0, 0 ),
-				to: new Date( 2026, 7, 30, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 7, 31, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 7, 30, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( last12Months, 'previous-period' ) ).toEqual( {
-				from: new Date( 2024, 7, 31, 0, 0, 0, 0 ),
-				to: new Date( 2025, 7, 30, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 7, 31, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 7, 30, 23, 59, 59, 999 ),
 			} );
 		} );
 	} );
@@ -314,25 +344,25 @@ describe( 'getComparisonRangeFromPreset', () => {
 	describe( 'previous-week', () => {
 		it( 'shifts a day-aligned range back seven days on day bounds', () => {
 			const yesterday = {
-				from: new Date( 2026, 7, 30, 0, 0, 0, 0 ),
-				to: new Date( 2026, 7, 30, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 7, 30, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 7, 30, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( yesterday, 'previous-week' ) ).toEqual( {
-				from: new Date( 2026, 7, 23, 0, 0, 0, 0 ),
-				to: new Date( 2026, 7, 23, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 7, 23, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 7, 23, 23, 59, 59, 999 ),
 			} );
 		} );
 
 		it( 'keeps the time of day for a rolling window', () => {
 			const rolling = {
-				from: new Date( 2026, 6, 9, 14, 30, 0, 0 ),
-				to: new Date( 2026, 6, 10, 14, 29, 59, 999 ),
+				from: siteDate( 2026, 6, 9, 14, 30, 0, 0 ),
+				to: siteDate( 2026, 6, 10, 14, 29, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( rolling, 'previous-week' ) ).toEqual( {
-				from: new Date( 2026, 6, 2, 14, 30, 0, 0 ),
-				to: new Date( 2026, 6, 3, 14, 29, 59, 999 ),
+				from: siteDate( 2026, 6, 2, 14, 30, 0, 0 ),
+				to: siteDate( 2026, 6, 3, 14, 29, 59, 999 ),
 			} );
 		} );
 
@@ -342,8 +372,8 @@ describe( 'getComparisonRangeFromPreset', () => {
 		 */
 		it( 'matches the previous period exactly at seven days', () => {
 			const week = {
-				from: new Date( 2026, 5, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 5, 7, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 5, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 5, 7, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( week, 'previous-week' ) ).toEqual(
@@ -359,14 +389,14 @@ describe( 'getComparisonRangeFromPreset', () => {
 			expect(
 				getComparisonRangeFromPreset(
 					{
-						from: new Date( 2025, 0, 1, 0, 0, 0, 0 ),
-						to: new Date( 2025, 11, 31, 23, 59, 59, 999 ),
+						from: siteDate( 2025, 0, 1, 0, 0, 0, 0 ),
+						to: siteDate( 2025, 11, 31, 23, 59, 59, 999 ),
 					},
 					'previous-period'
 				)
 			).toEqual( {
-				from: new Date( 2024, 0, 1, 0, 0, 0, 0 ),
-				to: new Date( 2024, 11, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 0, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2024, 11, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -374,14 +404,14 @@ describe( 'getComparisonRangeFromPreset', () => {
 			expect(
 				getComparisonRangeFromPreset(
 					{
-						from: new Date( 2025, 7, 20, 0, 0, 0, 0 ),
-						to: new Date( 2026, 7, 19, 23, 59, 59, 999 ),
+						from: siteDate( 2025, 7, 20, 0, 0, 0, 0 ),
+						to: siteDate( 2026, 7, 19, 23, 59, 59, 999 ),
 					},
 					'previous-period'
 				)
 			).toEqual( {
-				from: new Date( 2024, 7, 20, 0, 0, 0, 0 ),
-				to: new Date( 2025, 7, 19, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 7, 20, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 7, 19, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -390,12 +420,12 @@ describe( 'getComparisonRangeFromPreset', () => {
 			// back from 31 January clamps to 30 November: 62 days against the
 			// reference's 59, so the comparison counts days instead.
 			const clamping = {
-				from: new Date( 2026, 0, 31, 0, 0, 0, 0 ),
-				to: new Date( 2026, 2, 30, 23, 59, 59, 999 ),
+				from: siteDate( 2026, 0, 31, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 2, 30, 23, 59, 59, 999 ),
 			};
 			const expected = {
-				from: new Date( 2025, 11, 3, 0, 0, 0, 0 ),
-				to: new Date( 2026, 0, 30, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 11, 3, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 0, 30, 23, 59, 59, 999 ),
 			};
 
 			expect( getComparisonRangeFromPreset( clamping, 'previous-period' ) ).toEqual( expected );
@@ -407,14 +437,14 @@ describe( 'getComparisonRangeFromPreset', () => {
 			expect(
 				getComparisonRangeFromPreset(
 					{
-						from: new Date( 2026, 0, 1, 0, 0, 0, 0 ),
-						to: new Date( 2026, 1, 28, 23, 59, 59, 999 ),
+						from: siteDate( 2026, 0, 1, 0, 0, 0, 0 ),
+						to: siteDate( 2026, 1, 28, 23, 59, 59, 999 ),
 					},
 					'previous-period'
 				)
 			).toEqual( {
-				from: new Date( 2025, 10, 1, 0, 0, 0, 0 ),
-				to: new Date( 2025, 11, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 10, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 11, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 	} );
@@ -422,8 +452,8 @@ describe( 'getComparisonRangeFromPreset', () => {
 	describe( 'to-date presets', () => {
 		// `last-12-months` as read on 20 August 2026.
 		const reference = {
-			from: new Date( 2025, 8, 1, 0, 0, 0, 0 ),
-			to: new Date( 2026, 7, 20, 23, 59, 59, 999 ),
+			from: siteDate( 2025, 8, 1, 0, 0, 0, 0 ),
+			to: siteDate( 2026, 7, 20, 23, 59, 59, 999 ),
 		};
 
 		it( 'steps the previous period back by the completed window', () => {
@@ -433,7 +463,7 @@ describe( 'getComparisonRangeFromPreset', () => {
 				getComparisonRangeFromPreset( reference, 'previous-period', {
 					primaryPresetId: 'last-12-months',
 				} )?.from
-			).toEqual( new Date( 2024, 8, 1, 0, 0, 0, 0 ) );
+			).toEqual( siteDate( 2024, 8, 1, 0, 0, 0, 0 ) );
 		} );
 
 		it( 'stops the previous period as many days short as the reference does', () => {
@@ -444,7 +474,7 @@ describe( 'getComparisonRangeFromPreset', () => {
 				primaryPresetId: 'last-12-months',
 			} );
 
-			expect( comparison?.to ).toEqual( new Date( 2025, 7, 20, 23, 59, 59, 999 ) );
+			expect( comparison?.to ).toEqual( siteDate( 2025, 7, 20, 23, 59, 59, 999 ) );
 			expect( differenceInDays( comparison!.to!, comparison!.from! ) ).toBe(
 				differenceInDays( reference.to, reference.from )
 			);
@@ -458,16 +488,16 @@ describe( 'getComparisonRangeFromPreset', () => {
 					primaryPresetId: 'last-12-months',
 				} )
 			).toEqual( {
-				from: new Date( 2024, 8, 1, 0, 0, 0, 0 ),
-				to: new Date( 2025, 7, 20, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 8, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 7, 20, 23, 59, 59, 999 ),
 			} );
 			expect(
 				getComparisonRangeFromPreset( reference, 'previous-month', {
 					primaryPresetId: 'last-12-months',
 				} )
 			).toEqual( {
-				from: new Date( 2025, 7, 1, 0, 0, 0, 0 ),
-				to: new Date( 2026, 6, 20, 23, 59, 59, 999 ),
+				from: siteDate( 2025, 7, 1, 0, 0, 0, 0 ),
+				to: siteDate( 2026, 6, 20, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -476,8 +506,8 @@ describe( 'getComparisonRangeFromPreset', () => {
 			expect(
 				getComparisonRangeFromPreset( reference, 'previous-period', { primaryPresetId: 'custom' } )
 			).toEqual( {
-				from: new Date( 2024, 8, 12, 0, 0, 0, 0 ),
-				to: new Date( 2025, 7, 31, 23, 59, 59, 999 ),
+				from: siteDate( 2024, 8, 12, 0, 0, 0, 0 ),
+				to: siteDate( 2025, 7, 31, 23, 59, 59, 999 ),
 			} );
 		} );
 
@@ -485,13 +515,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 		// read on 8 March came back three days short, and year to date read on
 		// 1 January 2028 came back inverted.
 		it.each( [
-			[ 'March, after a shorter February', new Date( 2026, 2, 1 ), new Date( 2026, 2, 8 ) ],
-			[ 'February, after a longer January', new Date( 2026, 1, 1 ), new Date( 2026, 1, 15 ) ],
-			[ 'September, after a longer August', new Date( 2026, 8, 1 ), new Date( 2026, 8, 8 ) ],
+			[ 'March, after a shorter February', siteDate( 2026, 2, 1 ), siteDate( 2026, 2, 8 ) ],
+			[ 'February, after a longer January', siteDate( 2026, 1, 1 ), siteDate( 2026, 1, 15 ) ],
+			[ 'September, after a longer August', siteDate( 2026, 8, 1 ), siteDate( 2026, 8, 8 ) ],
 		] )( 'mirrors month to date with an equal-length window in %s', ( _label, first, last ) => {
 			const monthToDate = {
-				from: new Date( first.getFullYear(), first.getMonth(), first.getDate(), 0, 0, 0, 0 ),
-				to: new Date( last.getFullYear(), last.getMonth(), last.getDate(), 23, 59, 59, 999 ),
+				from: siteDate( first.getFullYear(), first.getMonth(), first.getDate(), 0, 0, 0, 0 ),
+				to: siteDate( last.getFullYear(), last.getMonth(), last.getDate(), 23, 59, 59, 999 ),
 			};
 
 			const comparison = getComparisonRangeFromPreset( monthToDate, 'previous-period', {
@@ -505,13 +535,13 @@ describe( 'getComparisonRangeFromPreset', () => {
 		} );
 
 		it.each( [
-			[ 'the first day of a leap year', new Date( 2028, 0, 1 ), new Date( 2028, 0, 1 ) ],
-			[ 'the first day after one', new Date( 2025, 0, 1 ), new Date( 2025, 0, 1 ) ],
-			[ 'a day well into a leap year', new Date( 2028, 0, 1 ), new Date( 2028, 2, 5 ) ],
+			[ 'the first day of a leap year', siteDate( 2028, 0, 1 ), siteDate( 2028, 0, 1 ) ],
+			[ 'the first day after one', siteDate( 2025, 0, 1 ), siteDate( 2025, 0, 1 ) ],
+			[ 'a day well into a leap year', siteDate( 2028, 0, 1 ), siteDate( 2028, 2, 5 ) ],
 		] )( 'mirrors year to date with an equal-length window on %s', ( _label, first, last ) => {
 			const yearToDate = {
-				from: new Date( first.getFullYear(), first.getMonth(), first.getDate(), 0, 0, 0, 0 ),
-				to: new Date( last.getFullYear(), last.getMonth(), last.getDate(), 23, 59, 59, 999 ),
+				from: siteDate( first.getFullYear(), first.getMonth(), first.getDate(), 0, 0, 0, 0 ),
+				to: siteDate( last.getFullYear(), last.getMonth(), last.getDate(), 23, 59, 59, 999 ),
 			};
 
 			const comparison = getComparisonRangeFromPreset( yearToDate, 'previous-period', {
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/site-datetime.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/site-datetime.test.ts
index 1de69eac4339..bc4155fb0935 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/site-datetime.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/site-datetime.test.ts
@@ -53,10 +53,15 @@ describe( 'parseSiteDateTime', () => {
 		expect( date?.toISOString() ).toBe( '2026-07-09T04:12:57.000Z' );
 	} );
 
-	it( 'accepts a Date instance unchanged', () => {
+	it( 'anchors a Date instance to the site timezone, keeping its instant', () => {
 		const source = new Date( '2026-06-29T12:00:00.000Z' );
+		const date = parseSiteDateTime( source );
 
-		expect( parseSiteDateTime( source ) ).toBe( source );
+		expect( date?.getTime() ).toBe( source.getTime() );
+		expect( date?.timeZone ).toBe( 'Europe/Amsterdam' );
+		// The zone is what the plain input could not carry: 12:00 UTC is 14:00
+		// on the site's clock, and a browser-zone read would name another hour.
+		expect( date?.getHours() ).toBe( 14 );
 	} );
 
 	it( 'returns undefined for a malformed value', () => {
diff --git a/projects/packages/premium-analytics/packages/datetime/src/date-range-span.ts b/projects/packages/premium-analytics/packages/datetime/src/date-range-span.ts
index c255619e65d5..13d0a0f77aba 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/date-range-span.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/date-range-span.ts
@@ -12,10 +12,11 @@ import {
 	isSameDay,
 	startOfDay,
 } from 'date-fns';
+import type { DateRange } from './get-comparison-range';
+import type { TZDate } from '@date-fns/tz';
 /**
  * Internal dependencies
  */
-import type { DateRange } from './get-comparison-range';
 
 /**
  * The unit a range's length is best described in.
@@ -58,7 +59,7 @@ const MONTHS_PER_YEAR = 12;
  * @param to   - Range end.
  * @return Whether both ends sit on a day boundary.
  */
-function coversWholeDays( from: Date, to: Date ): boolean {
+function coversWholeDays( from: TZDate, to: TZDate ): boolean {
 	return isEqual( from, startOfDay( from ) ) && isEqual( to, endOfDay( to ) );
 }
 
@@ -72,9 +73,7 @@ function coversWholeDays( from: Date, to: Date ): boolean {
  * @param to   - Range end.
  * @return The month count, or null when the range is not a whole number of months.
  */
-function getWholeMonths( from: Date, to: Date ): number | null {
-	// `addDays` keeps the input's `Date` subclass, so a site-timezone `TZDate`
-	// stays anchored to that zone rather than the browser's.
+function getWholeMonths( from: TZDate, to: TZDate ): number | null {
 	const dayAfterTo = startOfDay( addDays( to, 1 ) );
 	const months = differenceInCalendarMonths( dayAfterTo, from );
 
@@ -97,7 +96,7 @@ function getWholeMonths( from: Date, to: Date ): number | null {
  * open-ended range measures by the day it is read on.
  *
  * @param range - The range to measure.
- * @return The span, or null when the range is missing an end.
+ * @return The span, or null when the range is missing an end or runs backwards.
  */
 export function getDateRangeSpan( range?: DateRange ): DateRangeSpan | null {
 	const from = range?.from;
@@ -107,6 +106,13 @@ export function getDateRangeSpan( range?: DateRange ): DateRangeSpan | null {
 		return null;
 	}
 
+	// A hand-edited URL can name an end before its start. Every measurement
+	// below floors at 1, so a backwards range would otherwise report a
+	// plausible-looking one-hour window rather than no window at all.
+	if ( to.getTime() < from.getTime() ) {
+		return null;
+	}
+
 	if ( ! coversWholeDays( from, to ) ) {
 		// `round`, not the default truncation: an hour-snapped window runs to the
 		// last millisecond of its final hour, and truncation reads it an hour short.
diff --git a/projects/packages/premium-analytics/packages/datetime/src/drill-date-range.ts b/projects/packages/premium-analytics/packages/datetime/src/drill-date-range.ts
index 0c40e6ee79c0..de3211919b10 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/drill-date-range.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/drill-date-range.ts
@@ -16,6 +16,7 @@ import {
  */
 import type { DateRange } from './get-comparison-range';
 import type { IntervalType } from './interval';
+import type { TZDate } from '@date-fns/tz';
 
 /**
  * Bucket boundaries per interval a chart can draw.
@@ -24,7 +25,7 @@ import type { IntervalType } from './interval';
  * drawn in hours has nothing below it to open.
  */
 const BUCKET_BOUNDS: Partial<
-	Record< IntervalType, { start: ( date: Date ) => Date; end: ( date: Date ) => Date } >
+	Record< IntervalType, { start: ( date: TZDate ) => TZDate; end: ( date: TZDate ) => TZDate } >
 > = {
 	day: { start: startOfDay, end: endOfDay },
 	// ISO weeks, matching how the report's own week buckets are cut.
@@ -44,7 +45,11 @@ const BUCKET_BOUNDS: Partial<
  * @param now      - The current instant, for the clamp.
  * @return The bucket's range, or null when the interval has nothing below it.
  */
-export function drillDateRange( date: Date, interval: IntervalType, now: Date ): DateRange | null {
+export function drillDateRange(
+	date: TZDate,
+	interval: IntervalType,
+	now: TZDate
+): DateRange | null {
 	const bounds = BUCKET_BOUNDS[ interval ];
 
 	if ( ! bounds ) {
diff --git a/projects/packages/premium-analytics/packages/datetime/src/get-comparison-range.ts b/projects/packages/premium-analytics/packages/datetime/src/get-comparison-range.ts
index 6a8d2b78999b..f74771fb1742 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/get-comparison-range.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/get-comparison-range.ts
@@ -25,8 +25,15 @@ import {
  */
 import { completeToDateRange } from './to-date-range';
 import type { PrimaryPresetId } from './presets/types';
+import type { TZDate } from '@date-fns/tz';
 
-export type DateRange = { from?: Date; to?: Date };
+/**
+ * An inclusive range of instants, each anchored to the zone it was read in.
+ *
+ * Zoned rather than plain, so `getDateRangeSpan` cuts day boundaries on the
+ * site's clock; a plain `Date` cuts them on the browser's and lands a day out.
+ */
+export type DateRange = { from?: TZDate; to?: TZDate };
 
 export const COMPARISON_PREVIOUS_PERIOD = 'previous-period' as const;
 export const COMPARISON_PREVIOUS_WEEK = 'previous-week' as const;
@@ -62,7 +69,7 @@ export function isComparisonPresetId( value: unknown ): value is ComparisonPrese
  * @param to   - Range end.
  * @return The inclusive day count.
  */
-function getInclusiveDayCount( from: Date, to: Date ): number {
+function getInclusiveDayCount( from: TZDate, to: TZDate ): number {
 	return differenceInDays( to, from ) + 1;
 }
 
@@ -79,7 +86,7 @@ function getInclusiveDayCount( from: Date, to: Date ): number {
  * @param to   - Range end.
  * @return The month count, or null.
  */
-export function getWholeMonthCount( from: Date, to: Date ): number | null {
+export function getWholeMonthCount( from: TZDate, to: TZDate ): number | null {
 	const isDayAligned =
 		from.getTime() === startOfDay( from ).getTime() && to.getTime() === endOfDay( to ).getTime();
 
@@ -145,7 +152,7 @@ export function getComparisonRangeFromPreset(
 	// duration: a calendar shift clamps day-of-month and would collapse the window.
 	if ( ! isDayAligned ) {
 		const windowMs = differenceInMilliseconds( refTo, refFrom );
-		let to: Date;
+		let to: TZDate;
 
 		if ( presetId === COMPARISON_PREVIOUS_PERIOD ) {
 			// Both ends are inclusive, so the window lasts `windowMs + 1`; shifting
@@ -167,7 +174,9 @@ export function getComparisonRangeFromPreset(
 		};
 	}
 
-	const clampDayBound = ( date: Date, bound: 0 | 1 ) =>
+	// Annotated: a nested `date-fns` call has no contextual type to infer the
+	// zoned subclass from, and would widen the result back to a plain `Date`.
+	const clampDayBound = ( date: TZDate, bound: 0 | 1 ): TZDate =>
 		bound === 1 ? endOfDay( startOfDay( date ) ) : startOfDay( date );
 
 	if ( presetId === COMPARISON_PREVIOUS_PERIOD ) {
diff --git a/projects/packages/premium-analytics/packages/datetime/src/index.ts b/projects/packages/premium-analytics/packages/datetime/src/index.ts
index 388d53a40966..4bd5e13b942f 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/index.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/index.ts
@@ -6,6 +6,12 @@ export {
 } from './get-comparison-range';
 export type { ComparisonRangeOptions } from './get-comparison-range';
 
+/**
+ * Re-exported so a consumer naming a `DateRange` bound does not have to take a
+ * direct dependency on `@date-fns/tz`.
+ */
+export type { TZDate } from '@date-fns/tz';
+
 export {
 	createTZDateFromParts,
 	toLocalTZ,
diff --git a/projects/packages/premium-analytics/packages/datetime/src/presets/primary.ts b/projects/packages/premium-analytics/packages/datetime/src/presets/primary.ts
index 6b3393e098f7..7a29b3af59f6 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/presets/primary.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/presets/primary.ts
@@ -48,18 +48,19 @@ import {
 	type YearSurfacePresetId,
 } from './types';
 import type { DateRange } from '../get-comparison-range';
+import type { TZDate } from '@date-fns/tz';
 
 /**
  * Shared date calculations used by multiple presets.
  */
 type DateContext = {
-	now: Date;
-	initOfToday: Date;
-	endOfToday: Date;
-	endOfYesterday: Date;
-	lastMonth: Date;
-	endOfLastMonth: Date;
-	lastYear: Date;
+	now: TZDate;
+	initOfToday: TZDate;
+	endOfToday: TZDate;
+	endOfYesterday: TZDate;
+	lastMonth: TZDate;
+	endOfLastMonth: TZDate;
+	lastYear: TZDate;
 	timeZone: string;
 };
 
@@ -232,7 +233,9 @@ function buildDateContext( timeZone: string ): DateContext {
 	const nowWithTZ = toLocalTZ( undefined, timeZone );
 	const initOfToday = startOfDay( nowWithTZ );
 	const endOfToday = endOfDay( nowWithTZ );
-	const endOfYesterday = endOfDay( subDays( initOfToday, 1 ) );
+	// A nested `date-fns` call has no contextual type, and widens the result to `Date`.
+	const initOfYesterday = subDays( initOfToday, 1 );
+	const endOfYesterday = endOfDay( initOfYesterday );
 	const lastMonth = subMonths( initOfToday, 1 );
 	const endOfLastMonth = endOfMonth( lastMonth );
 	const lastYear = subYears( initOfToday, 1 );
diff --git a/projects/packages/premium-analytics/packages/datetime/src/site-datetime.ts b/projects/packages/premium-analytics/packages/datetime/src/site-datetime.ts
index aeaa95dfe552..f4f4848041de 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/site-datetime.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/site-datetime.ts
@@ -4,6 +4,7 @@
 import { reportingTimeZone } from './reporting-time-zone';
 import { readSiteTimestamp } from './site-timestamp';
 import { toLocalTZ } from './tz';
+import type { TZDate } from '@date-fns/tz';
 
 /**
  * Parse a timestamp in the timezone reports are read in.
@@ -14,9 +15,11 @@ import { toLocalTZ } from './tz';
  * @param value - The raw timestamp, or a `Date`.
  * @return The instant, or `undefined` when the value is missing or malformed.
  */
-export function parseSiteDateTime( value: unknown ): Date | undefined {
+export function parseSiteDateTime( value: unknown ): TZDate | undefined {
 	if ( value instanceof Date ) {
-		return isNaN( value.getTime() ) ? undefined : value;
+		// Re-anchored, not returned as-is: the same instant, read in the reporting
+		// zone whatever zone the caller's `Date` carried.
+		return isNaN( value.getTime() ) ? undefined : toLocalTZ( value.getTime(), reportingTimeZone() );
 	}
 
 	if ( typeof value !== 'string' ) {
@@ -31,6 +34,5 @@ export function parseSiteDateTime( value: unknown ): Date | undefined {
 
 	const parsed = toLocalTZ( timestamp.value, reportingTimeZone() );
 
-	// A plain `Date`, so callers keep reading its parts in their own zone as they did.
-	return isNaN( parsed.getTime() ) ? undefined : new Date( parsed.getTime() );
+	return isNaN( parsed.getTime() ) ? undefined : parsed;
 }
diff --git a/projects/packages/premium-analytics/packages/datetime/src/tz.ts b/projects/packages/premium-analytics/packages/datetime/src/tz.ts
index 3e46ba26c897..93fcdf5e3305 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/tz.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/tz.ts
@@ -163,9 +163,9 @@ export function dateToISOStringWithTZ( date: Date, timezone: string ): string {
  *
  * @param date     - The date to get the start of day for
  * @param timeZone - Timezone string (e.g., 'America/New_York', 'UTC', '+08:00')
- * @return A Date object representing midnight in the specified timezone
+ * @return A `TZDate` representing midnight in the specified timezone
  */
-export function startOfDayTZ( date: Date | number, timeZone: string ): Date {
+export function startOfDayTZ( date: Date | number, timeZone: string ): TZDate {
 	const tzDate = new TZDateMini( new Date( date ).getTime(), timeZone );
 	// startOfDay from date-fns respects the timezone context in TZDate
 	return startOfDay( tzDate );
@@ -176,9 +176,9 @@ export function startOfDayTZ( date: Date | number, timeZone: string ): Date {
  *
  * @param date     - The date to get the end of day for
  * @param timeZone - Timezone string (e.g., 'America/New_York', 'UTC', '+08:00')
- * @return A Date object representing the last millisecond of the day in the specified timezone
+ * @return A `TZDate` representing the last millisecond of the day in the specified timezone
  */
-export function endOfDayTZ( date: Date | number, timeZone: string ): Date {
+export function endOfDayTZ( date: Date | number, timeZone: string ): TZDate {
 	const tzDate = new TZDateMini( new Date( date ).getTime(), timeZone );
 	// endOfDay from date-fns respects the timezone context in TZDate
 	return endOfDay( tzDate );
diff --git a/projects/packages/premium-analytics/packages/formatters/README.md b/projects/packages/premium-analytics/packages/formatters/README.md
index a48022d9a891..a2326c9f0f22 100644
--- a/projects/packages/premium-analytics/packages/formatters/README.md
+++ b/projects/packages/premium-analytics/packages/formatters/README.md
@@ -154,10 +154,10 @@ formatDateRange( { from, to } ); // 'June 20 – 21, 2025'
 formatDateRange( { from, to }, { collapseSingleDay: true } ); // 'June 21, 2025'
 ```
 
-| Parameter                   | Type                         | Default | Description                                   |
-| --------------------------- | ---------------------------- | ------- | --------------------------------------------- |
-| `range`                     | `{ from?: Date; to?: Date }` |         | Date range object                             |
-| `options.collapseSingleDay` | `boolean`                    | `false` | Name a window of a day or less by its end day |
+| Parameter                   | Type                             | Default | Description                                   |
+| --------------------------- | -------------------------------- | ------- | --------------------------------------------- |
+| `range`                     | `{ from?: TZDate; to?: TZDate }` |         | Date range object                             |
+| `options.collapseSingleDay` | `boolean`                        | `false` | Name a window of a day or less by its end day |
 
 ## `formatDateRangeCompact( range? )` and `formatDateRangeMinimal( range? )`
 
@@ -228,11 +228,11 @@ formatDateRangeLong( { from, to }, { calendarScale: true } );
 // without the flag, the last of those reads 'Thursday, January 1 – Saturday, January 3'
 ```
 
-| Parameter               | Type                         | Default      | Description                                   |
-| ----------------------- | ---------------------------- | ------------ | --------------------------------------------- |
-| `range`                 | `{ from?: Date; to?: Date }` |              | Date range object                             |
-| `options.referenceYear` | `number`                     | current year | Year against which the year is redundant      |
-| `options.calendarScale` | `boolean`                    | `false`      | Force the calendar shape whatever it measures |
+| Parameter               | Type                             | Default      | Description                                   |
+| ----------------------- | -------------------------------- | ------------ | --------------------------------------------- |
+| `range`                 | `{ from?: TZDate; to?: TZDate }` |              | Date range object                             |
+| `options.referenceYear` | `number`                         | current year | Year against which the year is redundant      |
+| `options.calendarScale` | `boolean`                        | `false`      | Force the calendar shape whatever it measures |
 
 ## Implementation
 
diff --git a/projects/packages/premium-analytics/packages/formatters/src/date/__fixtures__/wp-date-settings.ts b/projects/packages/premium-analytics/packages/formatters/src/date/__fixtures__/wp-date-settings.ts
index d9a1333ef916..20c2db3f40ea 100644
--- a/projects/packages/premium-analytics/packages/formatters/src/date/__fixtures__/wp-date-settings.ts
+++ b/projects/packages/premium-analytics/packages/formatters/src/date/__fixtures__/wp-date-settings.ts
@@ -7,6 +7,7 @@
 /**
  * External dependencies
  */
+import { TZDate } from '@date-fns/tz';
 import { getSettings, type DateSettings } from '@wordpress/date';
 
 /**
@@ -99,5 +100,5 @@ export const ES_ES_SETTINGS = settingsFor(
  *               starts.
  * @return The date.
  */
-export const utcDate = ( year: number, month: number, day: number, hour: number = 0 ): Date =>
-	new Date( Date.UTC( year, month - 1, day, hour ) );
+export const utcDate = ( year: number, month: number, day: number, hour: number = 0 ): TZDate =>
+	new TZDate( Date.UTC( year, month - 1, day, hour ), 'UTC' );
diff --git a/projects/packages/premium-analytics/packages/formatters/src/date/__tests__/format-date-range.test.ts b/projects/packages/premium-analytics/packages/formatters/src/date/__tests__/format-date-range.test.ts
index e578916ebd8d..ff288341f59e 100644
--- a/projects/packages/premium-analytics/packages/formatters/src/date/__tests__/format-date-range.test.ts
+++ b/projects/packages/premium-analytics/packages/formatters/src/date/__tests__/format-date-range.test.ts
@@ -1,6 +1,7 @@
 /**
  * External dependencies
  */
+import { TZDate } from '@date-fns/tz';
 import { setSettings } from '@wordpress/date';
 import { resetLocaleData, setLocaleData } from '@wordpress/i18n';
 /**
@@ -89,7 +90,7 @@ describe( 'formatDateRange', () => {
 
 		it( 'falls back instead of throwing when one date is invalid', () => {
 			expect(
-				formatDateRange( { from: new Date( Number.NaN ), to: utcDate( 2025, 6, 21 ) } )
+				formatDateRange( { from: new TZDate( Number.NaN, 'UTC' ), to: utcDate( 2025, 6, 21 ) } )
 			).toBe( `Invalid date${ FALLBACK_SEP }June 21, 2025` );
 		} );
 	} );
diff --git a/projects/packages/premium-analytics/packages/routing/package.json b/projects/packages/premium-analytics/packages/routing/package.json
index 4bff1b340d51..900298827e12 100644
--- a/projects/packages/premium-analytics/packages/routing/package.json
+++ b/projects/packages/premium-analytics/packages/routing/package.json
@@ -13,5 +13,8 @@
 		"@wordpress/route": "0.20.0",
 		"date-fns": "4.1.0",
 		"react": "18.3.1"
+	},
+	"devDependencies": {
+		"@date-fns/tz": "1.4.1"
 	}
 }
diff --git a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/build-range-patch.test.ts b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/build-range-patch.test.ts
index 2e7e28e967f1..b70c657760ae 100644
--- a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/build-range-patch.test.ts
+++ b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/build-range-patch.test.ts
@@ -7,6 +7,10 @@ jest.mock( '@jetpack-premium-analytics/datetime', () => ( {
 	...jest.requireActual( '@jetpack-premium-analytics/datetime' ),
 	reportingTimeZone: () => '+00:00',
 } ) );
+/**
+ * External dependencies
+ */
+import { TZDate } from '@date-fns/tz';
 /**
  * Internal dependencies
  */
@@ -15,12 +19,12 @@ import { buildRangePatch } from '../build-range-patch';
 describe( 'buildRangePatch', () => {
 	// A rolling sub-day window: `to` sits mid-day, exactly where end-of-day
 	// rounding would corrupt it.
-	const from = new Date( '2026-07-09T14:30:00.000+00:00' );
-	const to = new Date( '2026-07-10T14:30:00.000+00:00' );
+	const from = new TZDate( '2026-07-09T14:30:00.000+00:00', 'UTC' );
+	const to = new TZDate( '2026-07-10T14:30:00.000+00:00', 'UTC' );
 
 	// A window long enough to allow day buckets, for the cases about carrying a
 	// selection rather than about coercing it.
-	const wideTo = new Date( '2026-07-19T14:30:00.000+00:00' );
+	const wideTo = new TZDate( '2026-07-19T14:30:00.000+00:00', 'UTC' );
 
 	it( 'returns null when there is nothing to stage', () => {
 		expect( buildRangePatch( { effective: {} } ) ).toBeNull();
@@ -121,7 +125,7 @@ describe( 'buildRangePatch', () => {
 	it( 'extends calendar and manual edits to the end of the day', () => {
 		// The end of the *site's* day (pinned to UTC above), whatever the host.
 		// A literal instant, so the expectation cannot drift with `endOfDayTZ`.
-		const expected = new Date( '2026-07-10T23:59:59.999+00:00' ).getTime();
+		const expected = new TZDate( '2026-07-10T23:59:59.999+00:00', 'UTC' ).getTime();
 
 		const custom = buildRangePatch( {
 			nextRange: { from, to },
@@ -166,8 +170,8 @@ describe( 'buildRangePatch', () => {
 	it( 'falls back to the previous period when the new range drops the preset', () => {
 		const patch = buildRangePatch( {
 			nextRange: {
-				from: new Date( '2026-08-01T00:00:00.000+00:00' ),
-				to: new Date( '2026-08-30T23:59:59.999+00:00' ),
+				from: new TZDate( '2026-08-01T00:00:00.000+00:00', 'UTC' ),
+				to: new TZDate( '2026-08-30T23:59:59.999+00:00', 'UTC' ),
 			},
 			nextPresetId: 'last-30-days',
 			effective: { comp: '1', compare_preset: 'previous-month' },
@@ -203,8 +207,8 @@ describe( 'buildRangePatch', () => {
 		// 12 September; as the to-date preset, twelve months back on the first.
 		const patch = buildRangePatch( {
 			nextRange: {
-				from: new Date( '2025-09-01T00:00:00.000Z' ),
-				to: new Date( '2026-08-20T23:59:59.999Z' ),
+				from: new TZDate( '2025-09-01T00:00:00.000Z', 'UTC' ),
+				to: new TZDate( '2026-08-20T23:59:59.999Z', 'UTC' ),
 			},
 			nextPresetId: 'last-12-months',
 			effective: { preset: 'last-7-days', comp: '1', compare_preset: 'previous-period' },
diff --git a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/use-report-date-filters.test.tsx b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/use-report-date-filters.test.tsx
index 26dfb8f8f329..77683362f9c5 100644
--- a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/use-report-date-filters.test.tsx
+++ b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/__tests__/use-report-date-filters.test.tsx
@@ -18,6 +18,7 @@ jest.mock( '@wordpress/route', () => ( {
 /**
  * External dependencies
  */
+import { TZDate } from '@date-fns/tz';
 import { act, renderHook } from '@testing-library/react';
 import { getSettings, setSettings } from '@wordpress/date';
 /**
@@ -102,8 +103,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-24T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-24T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'last-7-days'
 			);
@@ -138,8 +139,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-24T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-24T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'last-7-days'
 			);
@@ -162,8 +163,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onComparisonChange(
 				{
-					from: new Date( '2026-06-01T00:00:00.000Z' ),
-					to: new Date( '2026-06-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-06-01T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-06-30T23:59:59.999Z', 'UTC' ),
 				},
 				'previous-period'
 			);
@@ -255,8 +256,8 @@ describe( 'useReportDateFilters', () => {
 		act( () =>
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-10T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-10T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'custom'
 			)
@@ -264,8 +265,8 @@ describe( 'useReportDateFilters', () => {
 		act( () =>
 			result.current.onComparisonChange(
 				{
-					from: new Date( '2026-06-01T00:00:00.000Z' ),
-					to: new Date( '2026-06-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-06-01T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-06-30T23:59:59.999Z', 'UTC' ),
 				},
 				'previous-period'
 			)
@@ -274,8 +275,8 @@ describe( 'useReportDateFilters', () => {
 		act( () =>
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-01T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-01T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'custom'
 			)
@@ -319,8 +320,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-28T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-28T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'custom'
 			);
@@ -345,8 +346,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onChange(
 				{
-					from: new Date( '2026-07-24T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-24T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'last-7-days'
 			);
@@ -354,8 +355,8 @@ describe( 'useReportDateFilters', () => {
 		act( () => {
 			result.current.onComparisonChange(
 				{
-					from: new Date( '2026-06-01T00:00:00.000Z' ),
-					to: new Date( '2026-06-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-06-01T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-06-30T23:59:59.999Z', 'UTC' ),
 				},
 				'previous-period'
 			);
@@ -385,8 +386,8 @@ describe( 'useReportDateFilters', () => {
 		act( () =>
 			result.current.replaceRange(
 				{
-					from: new Date( '2026-07-24T00:00:00.000Z' ),
-					to: new Date( '2026-07-30T23:59:59.999Z' ),
+					from: new TZDate( '2026-07-24T00:00:00.000Z', 'UTC' ),
+					to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 				},
 				'last-7-days'
 			)
@@ -433,7 +434,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'day',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-07-21T13:45:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-21T13:45:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -452,7 +453,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'week',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-07-22T00:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-22T00:00:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -470,7 +471,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'month',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-02-14T00:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-02-14T00:00:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -488,7 +489,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'year',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2024-05-09T00:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2024-05-09T00:00:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -511,7 +512,9 @@ describe( 'useReportDateFilters', () => {
 				interval: 'year',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-02-14T00:00:00.000Z' ), 'month' ) );
+			act( () =>
+				result.current.drillDown( new TZDate( '2026-02-14T00:00:00.000Z', 'UTC' ), 'month' )
+			);
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -529,7 +532,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'day',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-07-21T13:45:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-21T13:45:00.000Z', 'UTC' ) ) );
 
 			expect( mockNavigate ).toHaveBeenCalledTimes( 1 );
 			expect( mockNavigate.mock.calls[ 0 ][ 0 ].replace ).toBeFalsy();
@@ -543,7 +546,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'hour',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-07-21T13:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-21T13:00:00.000Z', 'UTC' ) ) );
 
 			expect( mockNavigate ).not.toHaveBeenCalled();
 		} );
@@ -560,7 +563,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'day',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-09-05T00:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-09-05T00:00:00.000Z', 'UTC' ) ) );
 
 			expect( mockNavigate ).not.toHaveBeenCalled();
 		} );
@@ -574,7 +577,7 @@ describe( 'useReportDateFilters', () => {
 				interval: 'week',
 			} );
 
-			act( () => result.current.drillDown( new Date( '2026-07-23T00:00:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-23T00:00:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			expect( mockSearch ).toMatchObject( {
@@ -593,7 +596,7 @@ describe( 'useReportDateFilters', () => {
 
 			const cancelBeforeDrill = result.current.onCancel;
 
-			act( () => result.current.drillDown( new Date( '2026-07-21T13:45:00.000Z' ) ) );
+			act( () => result.current.drillDown( new TZDate( '2026-07-21T13:45:00.000Z', 'UTC' ) ) );
 			rerender();
 
 			act( () => cancelBeforeDrill() );
diff --git a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/use-report-date-filters.tsx b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/use-report-date-filters.tsx
index 1346e8a89642..1177133051ee 100644
--- a/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/use-report-date-filters.tsx
+++ b/projects/packages/premium-analytics/packages/routing/src/hooks/use-report-date-filters/use-report-date-filters.tsx
@@ -27,16 +27,14 @@ import type {
 	PrimaryPresetId,
 } from '@jetpack-premium-analytics/datetime';
 
-type PickerRange = { from: Date | undefined; to: Date | undefined };
-
 /**
  * The values and callbacks that drive `DateFiltersPanel`.
  */
 export type ReportDateFilters = {
 	presetId?: PrimaryPresetId;
-	range: PickerRange;
+	range: DateRange;
 	appliedPresetId?: PrimaryPresetId;
-	appliedRange: PickerRange;
+	appliedRange: DateRange;
 	comparisonPresetId?: ComparisonPresetId;
 	appliedComparisonPresetId?: ComparisonPresetId;
 
@@ -261,7 +259,11 @@ export function useReportDateFilters< TFrom extends string >( from?: TFrom ): Re
 			 * on the clock of the date passed in, and a plain instant would cut it
 			 * on the browser's clock instead.
 			 */
-			const drilled = drillDateRange( toLocalTZ( date, timeZone ), bucketInterval, new Date() );
+			const drilled = drillDateRange(
+				toLocalTZ( date, timeZone ),
+				bucketInterval,
+				toLocalTZ( undefined, timeZone )
+			);
 
 			if ( ! drilled?.from || ! drilled.to ) {
 				return;
diff --git a/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.test.ts b/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.test.ts
index 11c353def860..a1622171e434 100644
--- a/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.test.ts
+++ b/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.test.ts
@@ -1,6 +1,7 @@
 /**
  * External dependencies
  */
+import { TZDate } from '@date-fns/tz';
 import { getSettings, setSettings } from '@wordpress/date';
 /**
  * Internal dependencies
@@ -96,8 +97,8 @@ describe( 'encodeRangeToSearchParams', () => {
 	} );
 
 	// Midnight in the site zone, the shape the calendar inputs stage.
-	const from = new Date( '2026-06-29T04:00:00.000Z' );
-	const to = new Date( '2026-07-09T04:00:00.000Z' );
+	const from = new TZDate( '2026-06-29T04:00:00.000Z', 'America/New_York' );
+	const to = new TZDate( '2026-07-09T04:00:00.000Z', 'America/New_York' );
 
 	it( 'extends a calendar edit to the end of the site day', () => {
 		expect( encodeRangeToSearchParams( { from, to } ) ).toEqual( {
diff --git a/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.ts b/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.ts
index cb24456d0c93..f9919e33155e 100644
--- a/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.ts
+++ b/projects/packages/premium-analytics/packages/routing/src/search/date-range/date-range.ts
@@ -10,6 +10,7 @@ import {
 	reportingTimeZone,
 	type DateRange,
 	type PrimaryPresetId,
+	type TZDate,
 } from '@jetpack-premium-analytics/datetime';
 import { isValid } from 'date-fns';
 
@@ -20,7 +21,7 @@ import { isValid } from 'date-fns';
  * @param timezone - The timezone used by the picker.
  * @return The parsed date, or undefined when it is missing or malformed.
  */
-export function decodeDateSearchParam( value?: string, timezone?: string ): Date | undefined {
+export function decodeDateSearchParam( value?: string, timezone?: string ): TZDate | undefined {
 	if ( ! value ) {
 		return undefined;
 	}
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/__tests__/date-comparison-dropdown.test.tsx b/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/__tests__/date-comparison-dropdown.test.tsx
index 436e97563624..f5c83614fded 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/__tests__/date-comparison-dropdown.test.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/__tests__/date-comparison-dropdown.test.tsx
@@ -1,3 +1,4 @@
+import { TZDate } from '@date-fns/tz';
 import { render, screen } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
 import { DateComparisonDropdown } from '../date-comparison-dropdown';
@@ -8,13 +9,13 @@ const presets: ComparisonDateRangePreset[] = [
 		id: 'previous-period',
 		label: 'Previous period',
 		shortLabel: 'Prev. period',
-		range: { from: new Date( '2026-06-01' ), to: new Date( '2026-06-30' ) },
+		range: { from: new TZDate( '2026-06-01', 'UTC' ), to: new TZDate( '2026-06-30', 'UTC' ) },
 	},
 	{
 		id: 'previous-month',
 		label: 'Previous month',
 		shortLabel: 'Prev. month',
-		range: { from: new Date( '2026-05-01' ), to: new Date( '2026-05-31' ) },
+		range: { from: new TZDate( '2026-05-01', 'UTC' ), to: new TZDate( '2026-05-31', 'UTC' ) },
 	},
 ];
 
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/stories/date-comparison-dropdown.stories.tsx b/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/stories/date-comparison-dropdown.stories.tsx
index ed2640f092c5..98ad8db2edfd 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/stories/date-comparison-dropdown.stories.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-comparison-dropdown/stories/date-comparison-dropdown.stories.tsx
@@ -2,8 +2,7 @@ import { subDays, startOfDay, endOfDay } from 'date-fns';
 import { useState } from 'react';
 import { useComparisonDatePresets } from '../../use-comparison-date-presets';
 import { DateComparisonDropdown } from '../date-comparison-dropdown';
-import type { DateRange } from '../../date-range-popover';
-import type { ComparisonPresetId } from '@jetpack-premium-analytics/datetime';
+import type { DateRange, ComparisonPresetId } from '@jetpack-premium-analytics/datetime';
 import type { Meta, StoryObj } from '@storybook/react';
 
 const meta: Meta< typeof DateComparisonDropdown > = {
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-filters-panel/__tests__/date-filters-panel.test.tsx b/projects/packages/premium-analytics/packages/ui/src/date-filters-panel/__tests__/date-filters-panel.test.tsx
index f8d310fa5b2e..c882ee57503d 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-filters-panel/__tests__/date-filters-panel.test.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-filters-panel/__tests__/date-filters-panel.test.tsx
@@ -1,3 +1,4 @@
+import { TZDate } from '@date-fns/tz';
 import { ReportScopeProvider } from '@jetpack-premium-analytics/data';
 import { DETAIL_SURFACE_PRESETS } from '@jetpack-premium-analytics/datetime';
 import { render, screen, within } from '@testing-library/react';
@@ -6,8 +7,8 @@ import { DateFiltersPanel } from '../date-filters-panel';
 import type { ComponentProps } from 'react';
 
 const PRESET_RANGE = {
-	from: new Date( '2026-07-01T00:00:00.000Z' ),
-	to: new Date( '2026-07-30T23:59:59.999Z' ),
+	from: new TZDate( '2026-07-01T00:00:00.000Z', 'UTC' ),
+	to: new TZDate( '2026-07-30T23:59:59.999Z', 'UTC' ),
 };
 
 function panel( props: Partial< ComponentProps< typeof DateFiltersPanel > > = {} ) {
@@ -65,8 +66,8 @@ describe( 'DateFiltersPanel', () => {
 	// different ranges on screen at once (WOOA7S-1936).
 	it( 'names the applied preset on the trigger once one takes over', () => {
 		const customRange = {
-			from: new Date( '2026-01-30T00:00:00.000Z' ),
-			to: new Date( '2026-08-05T23:59:59.999Z' ),
+			from: new TZDate( '2026-01-30T00:00:00.000Z', 'UTC' ),
+			to: new TZDate( '2026-08-05T23:59:59.999Z', 'UTC' ),
 		};
 
 		const { rerender } = renderPanel( {
@@ -130,8 +131,8 @@ describe( 'DateFiltersPanel', () => {
 		// `last-12-months` as read on 20 August 2026. Measured by the day, the
 		// previous period would start on 12 September 2024.
 		const toDateRange = {
-			from: new Date( '2025-09-01T00:00:00.000Z' ),
-			to: new Date( '2026-08-20T23:59:59.999Z' ),
+			from: new TZDate( '2025-09-01T00:00:00.000Z', 'UTC' ),
+			to: new TZDate( '2026-08-20T23:59:59.999Z', 'UTC' ),
 		};
 		renderPanel( {
 			appliedPresetId: 'last-12-months',
@@ -145,8 +146,8 @@ describe( 'DateFiltersPanel', () => {
 
 		expect( onComparisonChange ).toHaveBeenCalledWith(
 			{
-				from: new Date( '2024-09-01T00:00:00.000Z' ),
-				to: new Date( '2025-08-20T23:59:59.999Z' ),
+				from: new TZDate( '2024-09-01T00:00:00.000Z', 'UTC' ),
+				to: new TZDate( '2025-08-20T23:59:59.999Z', 'UTC' ),
 			},
 			'previous-period'
 		);
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/__tests__/date-period-dropdown.test.tsx b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/__tests__/date-period-dropdown.test.tsx
index 78ad16be6d14..91c7c47abb60 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/__tests__/date-period-dropdown.test.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/__tests__/date-period-dropdown.test.tsx
@@ -3,14 +3,15 @@ jest.mock( '@wordpress/compose', () => ( {
 	useMediaQuery: jest.fn( () => false ),
 } ) );
 
+import { TZDate } from '@date-fns/tz';
 import { render, screen, within } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
 import { useMediaQuery } from '@wordpress/compose';
 import { DatePeriodDropdown } from '../date-period-dropdown';
 
 const JULY_2026 = {
-	from: new Date( 2026, 6, 1, 0, 0, 0, 0 ),
-	to: new Date( 2026, 6, 31, 23, 59, 59, 999 ),
+	from: new TZDate( 2026, 6, 1, 0, 0, 0, 0, 'UTC' ),
+	to: new TZDate( 2026, 6, 31, 23, 59, 59, 999, 'UTC' ),
 };
 
 function renderDropdown( overrides: Partial< Parameters< typeof DatePeriodDropdown >[ 0 ] > = {} ) {
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/date-period-dropdown.tsx b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/date-period-dropdown.tsx
index 6bfb7bf89fee..9f17e3af3ccc 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/date-period-dropdown.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/date-period-dropdown.tsx
@@ -5,6 +5,7 @@ import {
 	computePrimaryRange,
 	getMenuSurfacePresetGroups,
 	PRESET_CUSTOM,
+	type DateRange,
 	type PrimaryPresetId,
 	type QuickSurfacePresetId,
 } from '@jetpack-premium-analytics/datetime';
@@ -18,7 +19,7 @@ import { useCallback, useMemo, useRef, useState } from 'react';
 /**
  * Internal dependencies
  */
-import { DateRangePopoverContent, type DateRange } from '../date-range-popover';
+import { DateRangePopoverContent } from '../date-range-popover';
 import './date-period-dropdown.scss';
 
 /**
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/stories/date-period-dropdown.stories.tsx b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/stories/date-period-dropdown.stories.tsx
index 516061bc8555..e7d93289c697 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/stories/date-period-dropdown.stories.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-period-dropdown/stories/date-period-dropdown.stories.tsx
@@ -1,8 +1,11 @@
 import { DETAIL_SURFACE_PRESETS, computePrimaryRange } from '@jetpack-premium-analytics/datetime';
 import { useState } from 'react';
 import { DatePeriodDropdown } from '../date-period-dropdown';
-import type { DateRange } from '../../date-range-popover';
-import type { PrimaryPresetId, QuickSurfacePresetId } from '@jetpack-premium-analytics/datetime';
+import type {
+	DateRange,
+	PrimaryPresetId,
+	QuickSurfacePresetId,
+} from '@jetpack-premium-analytics/datetime';
 import type { Meta, StoryObj } from '@storybook/react';
 
 const meta: Meta< typeof DatePeriodDropdown > = {
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-range-input/date-range-input.tsx b/projects/packages/premium-analytics/packages/ui/src/date-range-input/date-range-input.tsx
index 52b5c660f834..aa56c12bf977 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-range-input/date-range-input.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-range-input/date-range-input.tsx
@@ -5,6 +5,7 @@ import {
 	createTZDateFromParts,
 	formatToTimezoneNaiveString,
 	getDatePart,
+	type TZDate,
 } from '@jetpack-premium-analytics/datetime';
 import { FormField, Input, Stack } from '@jetpack-premium-analytics/externals';
 import { __ } from '@wordpress/i18n';
@@ -24,8 +25,8 @@ type DateRangeInputProps = Pick<
 
 type DateInputProps = Pick< DateRangeInputProps, 'timeZone' > & {
 	label: string;
-	date?: Date;
-	onChange: ( date?: Date ) => void;
+	date?: TZDate;
+	onChange: ( date?: TZDate ) => void;
 };
 
 const formatToString = ( date: Date | undefined, timeZone: string ) =>
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/__tests__/date-range-filter.test.tsx b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/__tests__/date-range-filter.test.tsx
index a18ddb8f72f2..255e9e127b92 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/__tests__/date-range-filter.test.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/__tests__/date-range-filter.test.tsx
@@ -1,11 +1,13 @@
+import { TZDate } from '@date-fns/tz';
+import { type DateRange } from '@jetpack-premium-analytics/datetime';
 import { render, screen, within } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
 import { useState } from 'react';
-import { DateRangePopoverContent, type DateRange } from '../date-range-filter';
+import { DateRangePopoverContent } from '../date-range-filter';
 
 const JULY_2026: DateRange = {
-	from: new Date( 2026, 6, 1, 0, 0, 0, 0 ),
-	to: new Date( 2026, 6, 31, 23, 59, 59, 999 ),
+	from: new TZDate( 2026, 6, 1, 0, 0, 0, 0, 'UTC' ),
+	to: new TZDate( 2026, 6, 31, 23, 59, 59, 999, 'UTC' ),
 };
 
 type Props = Parameters< typeof DateRangePopoverContent >[ 0 ];
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/date-range-filter.tsx b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/date-range-filter.tsx
index 8137d053ea23..13e49673f502 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/date-range-filter.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/date-range-filter.tsx
@@ -1,7 +1,12 @@
 /**
  * External dependencies
  */
-import { PRESET_CUSTOM, type PrimaryPresetId } from '@jetpack-premium-analytics/datetime';
+import {
+	PRESET_CUSTOM,
+	toLocalTZ,
+	type DateRange,
+	type PrimaryPresetId,
+} from '@jetpack-premium-analytics/datetime';
 import { Button, RangeCalendar, Stack } from '@jetpack-premium-analytics/externals';
 import { __ } from '@wordpress/i18n';
 import clsx from 'clsx';
@@ -13,9 +18,11 @@ import { DateRangeInput } from '../date-range-input';
 import './date-range-filter.scss';
 
 /**
- * The calendar's own range type, from `@wordpress/ui`.
+ * The calendar's own range type, from `@wordpress/ui`. Its bounds are typed as
+ * plain `Date`, so a day leaves here through `toLocalTZ` to say in the type what
+ * the picker already does at runtime.
  */
-export type DateRange = NonNullable< Parameters< typeof RangeCalendar >[ 0 ][ 'value' ] >;
+type CalendarRange = NonNullable< Parameters< typeof RangeCalendar >[ 0 ][ 'value' ] >;
 
 type DateRangePopoverContentProps = {
 	range: DateRange;
@@ -106,22 +113,27 @@ export function DateRangePopoverContent( {
 	 * not the calendar's computed range: `RangeCalendar` never restarts a
 	 * complete range on click, it only moves the nearest endpoint.
 	 */
-	const handleCalendarChange = ( _nextRange: DateRange | null, triggerDate: Date ) => {
+	const handleCalendarChange = ( _nextRange: CalendarRange | null, triggerDate: Date ) => {
+		// The picker already builds its days in `timeZone`; this restates that in
+		// the type, since its `onValueChange` signature says plain `Date`.
+		const day = toLocalTZ( triggerDate, timeZone );
+
 		if ( draftRange?.from && ! draftRange.to ) {
 			const [ from, to ] =
-				triggerDate < draftRange.from
-					? [ triggerDate, draftRange.from ]
-					: [ draftRange.from, triggerDate ];
+				day < draftRange.from ? [ day, draftRange.from ] : [ draftRange.from, day ];
 
 			setDraftRange( null );
 			onChange( { from, to }, PRESET_CUSTOM );
 			return;
 		}
 
-		setDraftRange( { from: triggerDate, to: undefined } );
+		setDraftRange( { from: day, to: undefined } );
 	};
 
-	const calendarRange = draftRange ?? range;
+	// Widened to the calendar's shape, which requires both keys where a
+	// `DateRange` leaves them optional.
+	const selected = draftRange ?? range;
+	const calendarRange: CalendarRange = { from: selected.from, to: selected.to };
 
 	// Apply commits the staged range, not the draft: disable it mid-selection.
 	const effectiveCanApply = canApply && ! draftRange;
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/index.ts b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/index.ts
index 2e9543dd47b5..70e732905219 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/index.ts
+++ b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/index.ts
@@ -1,2 +1 @@
 export { DateRangePopoverContent } from './date-range-filter';
-export type { DateRange } from './date-range-filter';
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/stories/date-range-popover.stories.tsx b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/stories/date-range-popover.stories.tsx
index d5c309492045..d004483b2066 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-range-popover/stories/date-range-popover.stories.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-range-popover/stories/date-range-popover.stories.tsx
@@ -1,7 +1,8 @@
+import { toLocalTZ } from '@jetpack-premium-analytics/datetime';
 import { subDays, startOfDay, endOfDay } from 'date-fns';
 import { useState } from 'react';
 import { DateRangePopoverContent } from '../date-range-filter';
-import type { DateRange } from '../date-range-filter';
+import type { DateRange } from '@jetpack-premium-analytics/datetime';
 import type { Meta, StoryObj } from '@storybook/react';
 
 const meta: Meta< typeof DateRangePopoverContent > = {
@@ -20,15 +21,15 @@ export default meta;
 
 type Story = StoryObj< typeof DateRangePopoverContent >;
 
-const today = new Date();
+// Default timezone for Storybook - avoids dependency on WordPress stores
+const STORYBOOK_TIMEZONE = 'America/New_York';
+
+const today = toLocalTZ( undefined, STORYBOOK_TIMEZONE );
 const defaultRange: DateRange = {
 	from: startOfDay( subDays( today, 7 ) ),
 	to: endOfDay( subDays( today, 1 ) ),
 };
 
-// Default timezone for Storybook - avoids dependency on WordPress stores
-const STORYBOOK_TIMEZONE = 'America/New_York';
-
 function PopoverContentWithState( { isWideScreen = false } ) {
 	const [ range, setRange ] = useState< DateRange >( defaultRange );
 
diff --git a/projects/packages/premium-analytics/packages/ui/src/date-year-filter/date-year-filter.tsx b/projects/packages/premium-analytics/packages/ui/src/date-year-filter/date-year-filter.tsx
index b93e8198995a..242677a80530 100644
--- a/projects/packages/premium-analytics/packages/ui/src/date-year-filter/date-year-filter.tsx
+++ b/projects/packages/premium-analytics/packages/ui/src/date-year-filter/date-year-filter.tsx
@@ -4,6 +4,7 @@
 import {
 	computePrimaryRange,
 	getYearSurfacePresets,
+	type DateRange,
 	type DateRangePreset,
 	type PrimaryPresetId,
 	type YearSurfacePresetId,
@@ -17,7 +18,6 @@ import { useCallback, useLayoutEffect, useMemo, useState } from 'react';
 /**
  * Internal dependencies
  */
-import type { DateRange } from '../date-range-popover';
 import './date-year-filter.scss';
 
 export type DateYearFilterProps = {
diff --git a/projects/packages/premium-analytics/packages/ui/src/use-comparison-date-presets/use-comparison-date-presets.ts b/projects/packages/premium-analytics/packages/ui/src/use-comparison-date-presets/use-comparison-date-presets.ts
index aff114f74d58..8368f998e2a4 100644
--- a/projects/packages/premium-analytics/packages/ui/src/use-comparison-date-presets/use-comparison-date-presets.ts
+++ b/projects/packages/premium-analytics/packages/ui/src/use-comparison-date-presets/use-comparison-date-presets.ts
@@ -4,13 +4,10 @@
 import {
 	getComparisonOptions,
 	type ComparisonOption,
+	type DateRange,
 	type PrimaryPresetId,
 } from '@jetpack-premium-analytics/datetime';
 import { useMemo } from 'react';
-/**
- * Internal dependencies
- */
-import type { DateRange } from '../date-range-popover/date-range-filter';
 
 /**
  * A comparison option offered for the primary range, as the dropdown consumes
diff --git a/projects/packages/premium-analytics/packages/widgets-toolkit/src/components/report-page/__tests__/report-page-layout.test.tsx b/projects/packages/premium-analytics/packages/widgets-toolkit/src/components/report-page/__tests__/report-page-layout.test.tsx
index 9bb4d25b2e2d..edcb138e21e1 100644
--- a/projects/packages/premium-analytics/packages/widgets-toolkit/src/components/report-page/__tests__/report-page-layout.test.tsx
+++ b/projects/packages/premium-analytics/packages/widgets-toolkit/src/components/report-page/__tests__/report-page-layout.test.tsx
@@ -1,6 +1,7 @@
 /**
  * External dependencies
  */
+import { toLocalTZ } from '@jetpack-premium-analytics/datetime';
 import { DateFiltersPanel } from '@jetpack-premium-analytics/ui';
 import { render, screen } from '@testing-library/react';
 /**
@@ -18,14 +19,14 @@ jest.mock( '@jetpack-premium-analytics/ui', () => ( {
 const dateFiltersPanelMock = jest.mocked( DateFiltersPanel );
 
 const APPLIED_RANGE = {
-	from: new Date( Date.UTC( 2024, 0, 8 ) ),
-	to: new Date( Date.UTC( 2024, 0, 14, 23, 59, 59, 999 ) ),
+	from: toLocalTZ( Date.UTC( 2024, 0, 8 ), 'UTC' ),
+	to: toLocalTZ( Date.UTC( 2024, 0, 14, 23, 59, 59, 999 ), 'UTC' ),
 };
 
 // A draft over the applied window, so the controller reaches the panel mid-edit.
 const STAGED_RANGE = {
-	from: new Date( Date.UTC( 2019, 0, 7 ) ),
-	to: new Date( Date.UTC( 2019, 0, 13, 23, 59, 59, 999 ) ),
+	from: toLocalTZ( Date.UTC( 2019, 0, 7 ), 'UTC' ),
+	to: toLocalTZ( Date.UTC( 2019, 0, 13, 23, 59, 59, 999 ), 'UTC' ),
 };
 
 /** A controller mid-edit: a staged range and comparison over an applied window. */
diff --git a/projects/packages/premium-analytics/routes/detail-header.test.ts b/projects/packages/premium-analytics/routes/detail-header.test.ts
index ab79e75a9386..6d9f4d3412a5 100644
--- a/projects/packages/premium-analytics/routes/detail-header.test.ts
+++ b/projects/packages/premium-analytics/routes/detail-header.test.ts
@@ -1,6 +1,7 @@
 /**
  * External dependencies
  */
+import { TZDate } from '@date-fns/tz';
 import { getSettings, setSettings } from '@wordpress/date';
 /**
  * Internal dependencies
@@ -58,7 +59,7 @@ describe( 'performanceSentence', () => {
 		[ 'no range', undefined ],
 		[ 'an open start', { from: undefined, to: utcDate( 2026, 7, 15 ) } ],
 		[ 'an open end', { from: utcDate( 2026, 7, 9 ), to: undefined } ],
-		[ 'an unparseable bound', { from: new Date( 'nope' ), to: utcDate( 2026, 7, 15 ) } ],
+		[ 'an unparseable bound', { from: new TZDate( 'nope', 'UTC' ), to: utcDate( 2026, 7, 15 ) } ],
 	] )( 'states nothing for %s', ( _label, range ) => {
 		expect( performanceSentence( range ) ).toBeUndefined();
 	} );
diff --git a/projects/packages/premium-analytics/routes/post-detail/components/post-header-slots/post-header-slots.test.tsx b/projects/packages/premium-analytics/routes/post-detail/components/post-header-slots/post-header-slots.test.tsx
index 386136ea2e47..fbef30eaa8fd 100644
--- a/projects/packages/premium-analytics/routes/post-detail/components/post-header-slots/post-header-slots.test.tsx
+++ b/projects/packages/premium-analytics/routes/post-detail/components/post-header-slots/post-header-slots.test.tsx
@@ -1,3 +1,4 @@
+import { toLocalTZ } from '@jetpack-premium-analytics/datetime';
 import { SectionHeader } from '@jetpack-premium-analytics/ui';
 import { render, screen } from '@testing-library/react';
 import { postHeaderSlots } from './post-header-slots';
@@ -16,8 +17,8 @@ const SUMMARY: PostSummary = {
 // UTC-anchored: the sentence renders in the site zone, so a browser-local
 // `Date` would name the previous day in zones west of it.
 const PERFORMANCE_RANGE = {
-	from: new Date( Date.UTC( 2026, 6, 9 ) ),
-	to: new Date( Date.UTC( 2026, 6, 15 ) ),
+	from: toLocalTZ( Date.UTC( 2026, 6, 9 ), 'UTC' ),
+	to: toLocalTZ( Date.UTC( 2026, 6, 15 ), 'UTC' ),
 };
 
 /**
diff --git a/projects/packages/premium-analytics/routes/video-detail/components/video-header-slots/video-header-slots.test.tsx b/projects/packages/premium-analytics/routes/video-detail/components/video-header-slots/video-header-slots.test.tsx
index b36d786527b6..235d0c55ac16 100644
--- a/projects/packages/premium-analytics/routes/video-detail/components/video-header-slots/video-header-slots.test.tsx
+++ b/projects/packages/premium-analytics/routes/video-detail/components/video-header-slots/video-header-slots.test.tsx
@@ -1,3 +1,4 @@
+import { toLocalTZ } from '@jetpack-premium-analytics/datetime';
 import { SectionHeader } from '@jetpack-premium-analytics/ui';
 import { render, screen } from '@testing-library/react';
 import { videoHeaderSlots } from './video-header-slots';
@@ -16,8 +17,8 @@ const SUMMARY: VideoSummary = {
 // UTC-anchored: the sentence renders in the site zone, so a browser-local
 // `Date` would name the previous day in zones west of it.
 const PERFORMANCE_RANGE = {
-	from: new Date( Date.UTC( 2026, 6, 9 ) ),
-	to: new Date( Date.UTC( 2026, 6, 15 ) ),
+	from: toLocalTZ( Date.UTC( 2026, 6, 9 ), 'UTC' ),
+	to: toLocalTZ( Date.UTC( 2026, 6, 15 ), 'UTC' ),
 };
 
 /**