From 72704e2a56a1d659892c26259ff95d69e2887a30 Mon Sep 17 00:00:00 2001
From: Chi-Hsuan Huang <chihsuan.tw@gmail.com>
Date: Tue, 8 Sep 2026 16:41:30 +0800
Subject: [PATCH 1/4] refactor: converge the duplicated date-label validators
 and day-bound literals

---
 .../changelog/wooa7s-2079-dedup               |  4 +++
 .../src/processing/stats/bucket-window.ts     | 11 +++++--
 .../data/src/processing/stats/post.ts         | 11 +++----
 .../data/src/processing/stats/time-series.ts  | 33 +++++++++++--------
 .../data/src/processing/stats/utils.ts        | 10 +++---
 .../datetime/src/__tests__/date.test.ts       | 24 +++++++++++++-
 .../packages/datetime/src/date.ts             | 23 +++++++++++++
 .../packages/datetime/src/index.ts            |  3 ++
 .../popular-days/bucket-views-by-weekday.ts   | 15 ++-------
 .../widgets/post-views/use-post-views.ts      | 23 +++----------
 10 files changed, 98 insertions(+), 59 deletions(-)
 create mode 100644 projects/packages/premium-analytics/changelog/wooa7s-2079-dedup

diff --git a/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup b/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup
new file mode 100644
index 000000000000..6b63cc2e8dd3
--- /dev/null
+++ b/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup
@@ -0,0 +1,4 @@
+Significance: patch
+Type: changed
+
+Report dates: read every round-trip-validated date label and day bound through one helper.
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
index 365d84fe731d..829598c0aabf 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
@@ -12,7 +12,12 @@
 /**
  * External dependencies
  */
-import { formatDatePartWithTime, readSiteTimestamp } from '@jetpack-premium-analytics/datetime';
+import {
+	DAY_END_TIME,
+	DAY_START_TIME,
+	formatDatePartWithTime,
+	readSiteTimestamp,
+} from '@jetpack-premium-analytics/datetime';
 /**
  * Internal dependencies
  */
@@ -42,8 +47,8 @@ export function windowEndHour( value?: string ): number {
 const padTimePart = ( part: number ) => String( part ).padStart( 2, '0' );
 
 const EDGE_FALLBACKS = {
-	start: { time: '00:00:00', seconds: '00' },
-	end: { time: '23:59:59', seconds: '59' },
+	start: { time: DAY_START_TIME, seconds: '00' },
+	end: { time: DAY_END_TIME, seconds: '59' },
 } as const;
 
 // A window bound in the same timezone-naive wall-clock shape the bucket labels
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
index c51de305fccf..4b79c2afed2e 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
@@ -1,4 +1,4 @@
-import { format, isValid, parse } from 'date-fns';
+import { parseExactLabel } from '@jetpack-premium-analytics/datetime';
 import { safeParseFloat } from '../../utils/parsing';
 import { decodeHtmlText } from '../../utils/text';
 import { coerceStatsArray, coerceStatsRecord, isStatsRecord } from './utils';
@@ -99,12 +99,9 @@ const STATS_POST_DAY_FORMAT = 'yyyy-MM-dd';
 
 /** A real calendar day in the API's `YYYY-MM-DD` format. */
 function isValidStatsPostDay( value: string ): boolean {
-	if ( ! /^\d{4}-\d{2}-\d{2}$/.test( value ) ) {
-		return false;
-	}
-
-	const parsed = parse( value, STATS_POST_DAY_FORMAT, new Date( 0 ) );
-	return isValid( parsed ) && format( parsed, STATS_POST_DAY_FORMAT ) === value;
+	return (
+		/^\d{4}-\d{2}-\d{2}$/.test( value ) && parseExactLabel( value, STATS_POST_DAY_FORMAT ) !== null
+	);
 }
 
 function normalizeStatsPostYear( value: unknown ): StatsPostYear {
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
index fbb02dc806fc..542b21f3afe5 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
@@ -1,4 +1,10 @@
-import { formatDatePartWithTime, getDatePart } from '@jetpack-premium-analytics/datetime';
+import {
+	DAY_END_TIME,
+	DAY_START_TIME,
+	formatDatePartWithTime,
+	getDatePart,
+	parseExactLabel,
+} from '@jetpack-premium-analytics/datetime';
 import {
 	endOfISOWeek,
 	endOfMonth,
@@ -37,7 +43,6 @@ export type StatsTimeSeriesReport = StatsNormalizedReport & {
 
 const nonMetricFields = [ 'period', 'time_interval', 'date', 'date_start', 'date_end', 'hour' ];
 const dateFormat = 'yyyy-MM-dd';
-const referenceDate = new Date( 2001, 0, 1 );
 
 function numericTimeSeriesRow( row: StatsRecord ) {
 	return Object.fromEntries(
@@ -117,8 +122,8 @@ function getPrimaryMetricValue( row: StatsRecord ) {
 function getDateFnsIntervalFields( startDate: Date, endDate: Date ) {
 	return {
 		time_interval: format( startDate, dateFormat ),
-		date_start: formatDatePartWithTime( format( startDate, dateFormat ), '00:00:00' ),
-		date_end: formatDatePartWithTime( format( endDate, dateFormat ), '23:59:59' ),
+		date_start: formatDatePartWithTime( format( startDate, dateFormat ), DAY_START_TIME ),
+		date_end: formatDatePartWithTime( format( endDate, dateFormat ), DAY_END_TIME ),
 	};
 }
 
@@ -130,9 +135,9 @@ function getWeekIntervalFields( period: string ) {
 	}
 
 	const normalizedPeriod = `${ match[ 1 ] }-W${ match[ 2 ].padStart( 2, '0' ) }`;
-	const parsed = parse( normalizedPeriod, "RRRR-'W'II", referenceDate );
+	const parsed = parseExactLabel( normalizedPeriod, "RRRR-'W'II" );
 
-	if ( ! isValid( parsed ) || format( parsed, "RRRR-'W'II" ) !== normalizedPeriod ) {
+	if ( ! parsed ) {
 		return null;
 	}
 
@@ -150,8 +155,8 @@ function getWpcomWeekIntervalFields( period: string ) {
 
 	const parsed = parse(
 		`${ match[ 1 ] }-${ match[ 2 ] }-${ match[ 3 ] }`,
-		'yyyy-MM-dd',
-		referenceDate
+		dateFormat,
+		new Date( 0 )
 	);
 
 	if ( ! isValid( parsed ) ) {
@@ -162,9 +167,9 @@ function getWpcomWeekIntervalFields( period: string ) {
 }
 
 function getMonthIntervalFields( period: string ) {
-	const parsed = parse( period, 'yyyy-MM', referenceDate );
+	const parsed = parseExactLabel( period, 'yyyy-MM' );
 
-	if ( ! isValid( parsed ) || format( parsed, 'yyyy-MM' ) !== period ) {
+	if ( ! parsed ) {
 		return null;
 	}
 
@@ -172,9 +177,9 @@ function getMonthIntervalFields( period: string ) {
 }
 
 function getYearIntervalFields( period: string ) {
-	const parsed = parse( period, 'yyyy', referenceDate );
+	const parsed = parseExactLabel( period, 'yyyy' );
 
-	if ( ! isValid( parsed ) || format( parsed, 'yyyy' ) !== period ) {
+	if ( ! parsed ) {
 		return null;
 	}
 
@@ -317,8 +322,8 @@ export function sanitizeStatsTimeSeriesResponse(
 		summary: {
 			...getTimeSeriesSummarySidecars( response ),
 			...summary,
-			date_start: firstRow?.date_start ?? toSummaryBound( query?.start_date, '00:00:00' ),
-			date_end: lastRow?.date_end ?? toSummaryBound( query?.end_date ?? query?.date, '23:59:59' ),
+			date_start: firstRow?.date_start ?? toSummaryBound( query?.start_date, DAY_START_TIME ),
+			date_end: lastRow?.date_end ?? toSummaryBound( query?.end_date ?? query?.date, DAY_END_TIME ),
 		},
 		data,
 	};
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
index 9bdfb611b3c6..c863f13ea212 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
@@ -1,4 +1,6 @@
 import {
+	DAY_END_TIME,
+	DAY_START_TIME,
 	formatDatePartWithTime,
 	getDateIntervalDateParts,
 	getDatePart,
@@ -317,8 +319,8 @@ export function getStatsIntervalFields( date: string, period?: string ): StatsIn
 
 	return {
 		time_interval: date,
-		date_start: formatDatePartWithTime( startDate, '00:00:00' ),
-		date_end: formatDatePartWithTime( endDate, '23:59:59' ),
+		date_start: formatDatePartWithTime( startDate, DAY_START_TIME ),
+		date_end: formatDatePartWithTime( endDate, DAY_END_TIME ),
 	};
 }
 
@@ -337,8 +339,8 @@ export function getStatsSummaryIntervalFields(
 	const endDate = getStatsEndDateParam( query ) ?? responseDate ?? getDatePart( query?.start_date );
 
 	return {
-		...( startDate ? { date_start: formatDatePartWithTime( startDate, '00:00:00' ) } : {} ),
-		...( endDate ? { date_end: formatDatePartWithTime( endDate, '23:59:59' ) } : {} ),
+		...( startDate ? { date_start: formatDatePartWithTime( startDate, DAY_START_TIME ) } : {} ),
+		...( endDate ? { date_end: formatDatePartWithTime( endDate, DAY_END_TIME ) } : {} ),
 	};
 }
 
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
index fcb8f0558b06..ddde2b905575 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
@@ -1,7 +1,12 @@
 /**
  * Internal dependencies
  */
-import { formatDatePartWithTime, getDateIntervalDateParts, getDatePart } from '../date';
+import {
+	formatDatePartWithTime,
+	getDateIntervalDateParts,
+	getDatePart,
+	parseExactLabel,
+} from '../date';
 
 describe( 'date helpers', () => {
 	it( 'extracts date parts from ISO datetimes', () => {
@@ -45,4 +50,21 @@ describe( 'date helpers', () => {
 			endDate: '2026-12-31',
 		} );
 	} );
+
+	describe( 'parseExactLabel', () => {
+		it( 'parses a label that round-trips through its format', () => {
+			expect( parseExactLabel( '2026-06-22', 'yyyy-MM-dd' ) ).toEqual( new Date( 2026, 5, 22 ) );
+			expect( parseExactLabel( '2026-06', 'yyyy-MM' ) ).toEqual( new Date( 2026, 5, 1 ) );
+		} );
+
+		// date-fns rolls this forward to 2026-03-03, which `isValid` alone accepts.
+		it( 'rejects a day that does not exist', () => {
+			expect( parseExactLabel( '2026-02-31', 'yyyy-MM-dd' ) ).toBeNull();
+		} );
+
+		it( 'rejects a label written in another format', () => {
+			expect( parseExactLabel( '2026-6-22', 'yyyy-MM-dd' ) ).toBeNull();
+			expect( parseExactLabel( 'not a date', 'yyyy-MM-dd' ) ).toBeNull();
+		} );
+	} );
 } );
diff --git a/projects/packages/premium-analytics/packages/datetime/src/date.ts b/projects/packages/premium-analytics/packages/datetime/src/date.ts
index ad4ee2f941e0..392b9f0e9274 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/date.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/date.ts
@@ -12,6 +12,29 @@ export type DateIntervalDateParts = {
 
 const DATE_PART_FORMAT = 'yyyy-MM-dd';
 
+/** Inclusive day bounds, for the offset-less shape Stats responses carry. */
+export const DAY_START_TIME = '00:00:00';
+export const DAY_END_TIME = '23:59:59';
+
+// Only consulted for fields the parsed string omits, and every format we pass omits none.
+const REFERENCE_DATE = new Date( 2001, 0, 1 );
+
+/**
+ * Parse a label that must round-trip through its own format.
+ *
+ * date-fns rolls impossible values forward, so `2026-02-31` parses to 2026-03-03
+ * and `isValid` alone would accept it. Re-formatting catches that.
+ *
+ * @param label       - The label as written.
+ * @param labelFormat - The date-fns format the label must match exactly.
+ * @return The parsed date, or null when the label does not name a real one.
+ */
+export function parseExactLabel( label: string, labelFormat: string ): Date | null {
+	const parsed = parse( label, labelFormat, REFERENCE_DATE );
+
+	return isValid( parsed ) && format( parsed, labelFormat ) === label ? parsed : null;
+}
+
 /**
  * Extract the calendar date part from a date-like string.
  *
diff --git a/projects/packages/premium-analytics/packages/datetime/src/index.ts b/projects/packages/premium-analytics/packages/datetime/src/index.ts
index d29ae84a9ebf..74b12836b2da 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/index.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/index.ts
@@ -35,9 +35,12 @@ export { readSiteTimestamp, type SiteTimestamp, type TimestampParts } from './si
 export { reportingTimeZone, localTZDate, dateToISOStringWithLocalTZ } from './reporting-time-zone';
 
 export {
+	DAY_END_TIME,
+	DAY_START_TIME,
 	formatDatePartWithTime,
 	getDateIntervalDateParts,
 	getDatePart,
+	parseExactLabel,
 	type DateIntervalDateParts,
 	type DateIntervalPeriod,
 } from './date';
diff --git a/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts b/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
index 318d8f40fe1e..c6f3d2835b46 100644
--- a/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
+++ b/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
@@ -1,9 +1,9 @@
 /**
  * External dependencies
  */
-import { getDatePart } from '@jetpack-premium-analytics/datetime';
+import { getDatePart, parseExactLabel } from '@jetpack-premium-analytics/datetime';
 import { formatMondayFirstWeekday } from '@jetpack-premium-analytics/formatters';
-import { format, getDay, isValid, parse } from 'date-fns';
+import { getDay } from 'date-fns';
 
 export type PopularDayBucket = {
 	/** 0 = Monday … 6 = Sunday, matching the package's `weekStartsOn: 1` convention. */
@@ -17,9 +17,6 @@ export type PopularDayBucket = {
 
 const DATE_PART_FORMAT = 'yyyy-MM-dd';
 
-// Only consulted for fields the parsed string omits, and ours omits none.
-const referenceDate = new Date( 2001, 0, 1 );
-
 function weekdayLabel( weekday: number ) {
 	return formatMondayFirstWeekday( weekday );
 }
@@ -30,13 +27,7 @@ function weekdayLabel( weekday: number ) {
 function readRowDate( row: Record< string, unknown > ) {
 	const datePart = getDatePart( row.date_start ?? row.time_interval ?? row.period );
 
-	if ( ! datePart ) {
-		return undefined;
-	}
-
-	const parsed = parse( datePart, DATE_PART_FORMAT, referenceDate );
-
-	return isValid( parsed ) && format( parsed, DATE_PART_FORMAT ) === datePart ? parsed : undefined;
+	return datePart ? parseExactLabel( datePart, DATE_PART_FORMAT ) ?? undefined : undefined;
 }
 
 function readRowViews( row: Record< string, unknown > ) {
diff --git a/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts b/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
index 35fa89c271ee..dea166bb5ef2 100644
--- a/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
+++ b/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
@@ -8,6 +8,7 @@ import {
 	type StatsPostDay,
 } from '@jetpack-premium-analytics/data';
 import { parseSiteDateTime } from '@jetpack-premium-analytics/datetime';
+import { toDay } from '@jetpack-premium-analytics/widgets-toolkit';
 import { useMemo } from '@wordpress/element';
 import {
 	addDays,
@@ -57,34 +58,20 @@ type BucketWindow = {
 	to: string;
 };
 
-/**
- * Extract a validated `YYYY-MM-DD` day — validated because `bucketDays()` feeds
- * it to `parseISO()`/`each*OfInterval()`, which throw on invalid dates.
- */
-function toValidDay( value?: string ): string | undefined {
-	const day = value?.slice( 0, 10 );
-
-	if ( ! day || ! /^\d{4}-\d{2}-\d{2}$/.test( day ) || Number.isNaN( parseISO( day ).getTime() ) ) {
-		return undefined;
-	}
-
-	return day;
-}
-
 /**
  * Extract a `YYYY-MM-DD` window from ISO report params, or undefined when
  * either bound is missing/malformed. The endpoint's day keys are date-only,
  * so comparing date prefixes keeps the slice timezone-stable.
  */
 function toDayWindow( from?: string, to?: string ): DayWindow | undefined {
-	const fromDay = toValidDay( from );
-	const toDay = toValidDay( to );
+	const fromDay = toDay( from );
+	const toBound = toDay( to );
 
-	if ( ! fromDay || ! toDay ) {
+	if ( ! fromDay || ! toBound ) {
 		return undefined;
 	}
 
-	return { from: fromDay, to: toDay };
+	return { from: fromDay, to: toBound };
 }
 
 /**

From 9bb650fd41d603acff343c8fd4221bcfc954cb90 Mon Sep 17 00:00:00 2001
From: Chi-Hsuan Huang <chihsuan.tw@gmail.com>
Date: Tue, 8 Sep 2026 16:52:19 +0800
Subject: [PATCH 2/4] fix: correct the reference-date rationale and keep day
 bounds out of the datetime API

---
 .../data/src/processing/stats/bucket-window.ts        |  8 ++------
 .../packages/data/src/processing/stats/time-series.ts |  4 ++--
 .../packages/data/src/processing/stats/utils.ts       |  6 ++++--
 .../packages/datetime/src/__tests__/date.test.ts      |  8 +++++---
 .../premium-analytics/packages/datetime/src/date.ts   | 11 ++++-------
 .../premium-analytics/packages/datetime/src/index.ts  |  2 --
 .../src/helpers/__tests__/to-day.test.ts              |  9 ++-------
 7 files changed, 19 insertions(+), 29 deletions(-)

diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
index 829598c0aabf..67b30f259872 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
@@ -12,12 +12,8 @@
 /**
  * External dependencies
  */
-import {
-	DAY_END_TIME,
-	DAY_START_TIME,
-	formatDatePartWithTime,
-	readSiteTimestamp,
-} from '@jetpack-premium-analytics/datetime';
+import { formatDatePartWithTime, readSiteTimestamp } from '@jetpack-premium-analytics/datetime';
+import { DAY_END_TIME, DAY_START_TIME } from './utils';
 /**
  * Internal dependencies
  */
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
index 542b21f3afe5..8974193d9009 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
@@ -1,6 +1,4 @@
 import {
-	DAY_END_TIME,
-	DAY_START_TIME,
 	formatDatePartWithTime,
 	getDatePart,
 	parseExactLabel,
@@ -21,6 +19,8 @@ import { createStatsBucketWindowFilter, type StatsBucketFilter } from './bucket-
 import {
 	coerceStatsArray,
 	coerceStatsRecord,
+	DAY_END_TIME,
+	DAY_START_TIME,
 	getStatsIntervalFields,
 	normalizeStatsSummary,
 } from './utils';
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
index c863f13ea212..798302a5ecb3 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts
@@ -1,6 +1,4 @@
 import {
-	DAY_END_TIME,
-	DAY_START_TIME,
 	formatDatePartWithTime,
 	getDateIntervalDateParts,
 	getDatePart,
@@ -16,6 +14,10 @@ import type {
 } from './types';
 import type { StatsQueryParams } from '../../utils/stats-params';
 
+/** Inclusive day bounds, in the offset-less second-precision shape Stats responses carry. */
+export const DAY_START_TIME = '00:00:00';
+export const DAY_END_TIME = '23:59:59';
+
 type StatsComparisonKey = string | number;
 
 type StatsComparisonEntry< TComparison > = {
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
index ddde2b905575..754663ab7a1b 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
@@ -57,14 +57,16 @@ describe( 'date helpers', () => {
 			expect( parseExactLabel( '2026-06', 'yyyy-MM' ) ).toEqual( new Date( 2026, 5, 1 ) );
 		} );
 
-		// date-fns rolls this forward to 2026-03-03, which `isValid` alone accepts.
 		it( 'rejects a day that does not exist', () => {
 			expect( parseExactLabel( '2026-02-31', 'yyyy-MM-dd' ) ).toBeNull();
+			expect( parseExactLabel( 'not a date', 'yyyy-MM-dd' ) ).toBeNull();
 		} );
 
-		it( 'rejects a label written in another format', () => {
+		// Both parse to a real date that `isValid` accepts; only the round trip
+		// rejects them. 2025-W53 resolves into 2026, and the loose day into June.
+		it( 'rejects a label that parses to a different label', () => {
+			expect( parseExactLabel( '2025-W53', "RRRR-'W'II" ) ).toBeNull();
 			expect( parseExactLabel( '2026-6-22', 'yyyy-MM-dd' ) ).toBeNull();
-			expect( parseExactLabel( 'not a date', 'yyyy-MM-dd' ) ).toBeNull();
 		} );
 	} );
 } );
diff --git a/projects/packages/premium-analytics/packages/datetime/src/date.ts b/projects/packages/premium-analytics/packages/datetime/src/date.ts
index 392b9f0e9274..5c0d3c056b59 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/date.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/date.ts
@@ -12,18 +12,15 @@ export type DateIntervalDateParts = {
 
 const DATE_PART_FORMAT = 'yyyy-MM-dd';
 
-/** Inclusive day bounds, for the offset-less shape Stats responses carry. */
-export const DAY_START_TIME = '00:00:00';
-export const DAY_END_TIME = '23:59:59';
-
-// Only consulted for fields the parsed string omits, and every format we pass omits none.
+// date-fns needs a reference date. Every format here carries a year, which resets
+// the rest; a year-less format would silently inherit 2001.
 const REFERENCE_DATE = new Date( 2001, 0, 1 );
 
 /**
  * Parse a label that must round-trip through its own format.
  *
- * date-fns rolls impossible values forward, so `2026-02-31` parses to 2026-03-03
- * and `isValid` alone would accept it. Re-formatting catches that.
+ * `isValid` alone is not enough: date-fns reads `2025-W53` as a real date in 2026,
+ * and accepts a loosely written `2026-6-22`. Re-formatting catches both.
  *
  * @param label       - The label as written.
  * @param labelFormat - The date-fns format the label must match exactly.
diff --git a/projects/packages/premium-analytics/packages/datetime/src/index.ts b/projects/packages/premium-analytics/packages/datetime/src/index.ts
index 74b12836b2da..29cb8c50b57b 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/index.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/index.ts
@@ -35,8 +35,6 @@ export { readSiteTimestamp, type SiteTimestamp, type TimestampParts } from './si
 export { reportingTimeZone, localTZDate, dateToISOStringWithLocalTZ } from './reporting-time-zone';
 
 export {
-	DAY_END_TIME,
-	DAY_START_TIME,
 	formatDatePartWithTime,
 	getDateIntervalDateParts,
 	getDatePart,
diff --git a/projects/packages/premium-analytics/packages/widgets-toolkit/src/helpers/__tests__/to-day.test.ts b/projects/packages/premium-analytics/packages/widgets-toolkit/src/helpers/__tests__/to-day.test.ts
index 2cfe93c6a5a4..35b7dbcd4690 100644
--- a/projects/packages/premium-analytics/packages/widgets-toolkit/src/helpers/__tests__/to-day.test.ts
+++ b/projects/packages/premium-analytics/packages/widgets-toolkit/src/helpers/__tests__/to-day.test.ts
@@ -26,13 +26,8 @@ describe( 'toDay', () => {
 		expect( toDay( 'not-a-date' ) ).toBeUndefined();
 	} );
 
-	// The reason the stricter of the two former copies is the one hoisted: it
-	// protects two callers for two different reasons. `post-traffic-activity`
-	// feeds the result to parseISO/eachDayOfInterval, which throw on a
-	// well-shaped but non-existent day. `post-detail-highlights` only compares
-	// days as strings, so it wouldn't throw — but the loose check let a bad
-	// `from` still lexically match real days, producing a plausible-looking
-	// windowed sum instead of the documented all-time fallback.
+	// Callers feed the result to date maths that throws on an impossible day, or
+	// compare it as a string, where a loose check would lexically match real days.
 	it( 'returns undefined for a well-shaped but impossible calendar date', () => {
 		expect( toDay( '2026-02-31' ) ).toBeUndefined();
 		expect( toDay( '2026-13-01' ) ).toBeUndefined();

From da233adb0ffb7ace450b8debac63e19902a8611f Mon Sep 17 00:00:00 2001
From: Chi-Hsuan Huang <chihsuan.tw@gmail.com>
Date: Wed, 9 Sep 2026 13:20:06 +0800
Subject: [PATCH 3/4] refactor: converge the last inline date parse and pin the
 swapped guards with tests

---
 .../src/processing/stats/bucket-window.ts     |  2 +-
 .../data/src/processing/stats/post.ts         |  4 +---
 .../data/src/processing/stats/time-series.ts  | 10 ++------
 .../datetime/src/__tests__/date.test.ts       |  6 +++++
 .../packages/datetime/src/date.ts             |  2 +-
 .../popular-days/bucket-views-by-weekday.ts   |  2 +-
 .../__tests__/use-post-views.test.ts          | 24 +++++++++++++++++++
 .../widgets/post-views/use-post-views.ts      |  2 +-
 8 files changed, 37 insertions(+), 15 deletions(-)
 create mode 100644 projects/packages/premium-analytics/widgets/post-views/__tests__/use-post-views.test.ts

diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
index 67b30f259872..d283730fae64 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/bucket-window.ts
@@ -13,10 +13,10 @@
  * External dependencies
  */
 import { formatDatePartWithTime, readSiteTimestamp } from '@jetpack-premium-analytics/datetime';
-import { DAY_END_TIME, DAY_START_TIME } from './utils';
 /**
  * Internal dependencies
  */
+import { DAY_END_TIME, DAY_START_TIME } from './utils';
 import type { StatsQueryParams } from '../../utils/stats-params';
 
 // Bare dates and T-separated datetimes only — getDatePart splits on T alone
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
index 4b79c2afed2e..46d0c91e8c93 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts
@@ -99,9 +99,7 @@ const STATS_POST_DAY_FORMAT = 'yyyy-MM-dd';
 
 /** A real calendar day in the API's `YYYY-MM-DD` format. */
 function isValidStatsPostDay( value: string ): boolean {
-	return (
-		/^\d{4}-\d{2}-\d{2}$/.test( value ) && parseExactLabel( value, STATS_POST_DAY_FORMAT ) !== null
-	);
+	return parseExactLabel( value, STATS_POST_DAY_FORMAT ) !== null;
 }
 
 function normalizeStatsPostYear( value: unknown ): StatsPostYear {
diff --git a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
index 8974193d9009..812790c2495c 100644
--- a/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
+++ b/projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts
@@ -8,8 +8,6 @@ import {
 	endOfMonth,
 	endOfYear,
 	format,
-	isValid,
-	parse,
 	startOfISOWeek,
 	startOfMonth,
 	startOfYear,
@@ -153,13 +151,9 @@ function getWpcomWeekIntervalFields( period: string ) {
 		return null;
 	}
 
-	const parsed = parse(
-		`${ match[ 1 ] }-${ match[ 2 ] }-${ match[ 3 ] }`,
-		dateFormat,
-		new Date( 0 )
-	);
+	const parsed = parseExactLabel( `${ match[ 1 ] }-${ match[ 2 ] }-${ match[ 3 ] }`, dateFormat );
 
-	if ( ! isValid( parsed ) ) {
+	if ( ! parsed ) {
 		return null;
 	}
 
diff --git a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
index 754663ab7a1b..133df275b930 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/__tests__/date.test.ts
@@ -68,5 +68,11 @@ describe( 'date helpers', () => {
 			expect( parseExactLabel( '2025-W53', "RRRR-'W'II" ) ).toBeNull();
 			expect( parseExactLabel( '2026-6-22', 'yyyy-MM-dd' ) ).toBeNull();
 		} );
+
+		// The round trip still succeeds, so a year-less format resolves against the
+		// reference year rather than failing. Every caller's format must carry one.
+		it( 'anchors a year-less format to the 2001 reference year', () => {
+			expect( parseExactLabel( '06-22', 'MM-dd' ) ).toEqual( new Date( 2001, 5, 22 ) );
+		} );
 	} );
 } );
diff --git a/projects/packages/premium-analytics/packages/datetime/src/date.ts b/projects/packages/premium-analytics/packages/datetime/src/date.ts
index 5c0d3c056b59..0b8e47d45250 100644
--- a/projects/packages/premium-analytics/packages/datetime/src/date.ts
+++ b/projects/packages/premium-analytics/packages/datetime/src/date.ts
@@ -23,7 +23,7 @@ const REFERENCE_DATE = new Date( 2001, 0, 1 );
  * and accepts a loosely written `2026-6-22`. Re-formatting catches both.
  *
  * @param label       - The label as written.
- * @param labelFormat - The date-fns format the label must match exactly.
+ * @param labelFormat - The date-fns format the label must match exactly. Must carry a year.
  * @return The parsed date, or null when the label does not name a real one.
  */
 export function parseExactLabel( label: string, labelFormat: string ): Date | null {
diff --git a/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts b/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
index c6f3d2835b46..b9be173daf22 100644
--- a/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
+++ b/projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts
@@ -27,7 +27,7 @@ function weekdayLabel( weekday: number ) {
 function readRowDate( row: Record< string, unknown > ) {
 	const datePart = getDatePart( row.date_start ?? row.time_interval ?? row.period );
 
-	return datePart ? parseExactLabel( datePart, DATE_PART_FORMAT ) ?? undefined : undefined;
+	return datePart ? parseExactLabel( datePart, DATE_PART_FORMAT ) : null;
 }
 
 function readRowViews( row: Record< string, unknown > ) {
diff --git a/projects/packages/premium-analytics/widgets/post-views/__tests__/use-post-views.test.ts b/projects/packages/premium-analytics/widgets/post-views/__tests__/use-post-views.test.ts
new file mode 100644
index 000000000000..2c6f62406f75
--- /dev/null
+++ b/projects/packages/premium-analytics/widgets/post-views/__tests__/use-post-views.test.ts
@@ -0,0 +1,24 @@
+/**
+ * Internal dependencies
+ */
+import { toDayWindow } from '../use-post-views';
+
+describe( 'toDayWindow', () => {
+	it( 'slices the date part off both ISO bounds', () => {
+		expect(
+			toDayWindow( '2026-07-01T00:00:00.000+08:00', '2026-07-07T23:59:59.999+08:00' )
+		).toEqual( { from: '2026-07-01', to: '2026-07-07' } );
+	} );
+
+	it( 'returns undefined when a bound is missing', () => {
+		expect( toDayWindow( undefined, '2026-07-07T23:59:59.999+08:00' ) ).toBeUndefined();
+		expect( toDayWindow( '2026-07-01T00:00:00.000+08:00', undefined ) ).toBeUndefined();
+	} );
+
+	it( 'returns undefined for a well-shaped but impossible day', () => {
+		expect(
+			toDayWindow( '2026-02-31T00:00:00.000+08:00', '2026-07-07T23:59:59.999+08:00' )
+		).toBeUndefined();
+		expect( toDayWindow( '2026-07-01T00:00:00.000+08:00', 'not-a-date' ) ).toBeUndefined();
+	} );
+} );
diff --git a/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts b/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
index dea166bb5ef2..afc1cfc5a785 100644
--- a/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
+++ b/projects/packages/premium-analytics/widgets/post-views/use-post-views.ts
@@ -63,7 +63,7 @@ type BucketWindow = {
  * either bound is missing/malformed. The endpoint's day keys are date-only,
  * so comparing date prefixes keeps the slice timezone-stable.
  */
-function toDayWindow( from?: string, to?: string ): DayWindow | undefined {
+export function toDayWindow( from?: string, to?: string ): DayWindow | undefined {
 	const fromDay = toDay( from );
 	const toBound = toDay( to );
 

From 2e39381ab8ef9e9362e9dd2e517aa4ddcb5ca506 Mon Sep 17 00:00:00 2001
From: Chi-Hsuan Huang <chihsuan.tw@gmail.com>
Date: Thu, 10 Sep 2026 10:38:21 +0800
Subject: [PATCH 4/4] chore: file the changelog entry as an internal-refactor
 comment

---
 .../packages/premium-analytics/changelog/wooa7s-2079-dedup     | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup b/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup
index 6b63cc2e8dd3..80ddcd396f0c 100644
--- a/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup
+++ b/projects/packages/premium-analytics/changelog/wooa7s-2079-dedup
@@ -1,4 +1,3 @@
 Significance: patch
 Type: changed
-
-Report dates: read every round-trip-validated date label and day bound through one helper.
+Comment: Internal refactor: read the round-trip-validated date labels and day bounds through one shared helper.