test: verify SDK timestamps use UTC

[marandaneto]

💡 Motivation and Context

PostHog.bare_capture/3 adds an SDK-generated event timestamp in UTC. This test protects that behavior while confirming that a DateTime supplied as an event property keeps its original time zone offset.

The regression test captures an event with an Asia/Kathmandu property. It verifies that the generated event timestamp ends in Z and parses with a zero UTC offset. It also verifies that the captured property remains the original DateTime value and that JSON encoding produces 2024-01-02T03:04:05.678+05:45 for that property.

💚 How did you test it?

• mix test test/posthog_test.exs - 28 tests passed.
• mix test - 332 tests passed and 18 integration tests were excluded.
• mix format --check-formatted test/posthog_test.exs
• git diff --check origin/main...HEAD

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

No changeset was added because this PR only adds regression coverage and does not release a product change.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Pi's worker agent preserved the existing test-only change, ran the repository tests, and completed an isolated autoreview. The task was kept to regression coverage with no production or release metadata changes. A shareable agent session link was not available in this environment.

[github-actions]

posthog-elixir Compliance Report

Date: 2026-08-14 06:10:23 UTC
Duration: 117648ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	611ms
Format Validation.Event Has Uuid	✅	609ms
Format Validation.Event Has Lib Properties	✅	610ms
Format Validation.Distinct Id Is String	✅	610ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	610ms
Format Validation.Event Has Timestamp	✅	610ms
Retry Behavior.Retries On 503	✅	5615ms
Retry Behavior.Does Not Retry On 400	✅	2613ms
Retry Behavior.Does Not Retry On 401	✅	2613ms
Retry Behavior.Respects Retry After Header	✅	5616ms
Retry Behavior.Implements Backoff	✅	15626ms
Retry Behavior.Retries On 500	✅	5615ms
Retry Behavior.Retries On 502	✅	5616ms
Retry Behavior.Retries On 504	✅	5616ms
Retry Behavior.Max Retries Respected	✅	15626ms
Deduplication.Generates Unique Uuids	✅	622ms
Deduplication.Preserves Uuid On Retry	✅	5615ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10621ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	614ms
Deduplication.Different Events Have Different Uuids	✅	613ms
Compression.Sends Gzip When Enabled	✅	609ms
Batch Format.Uses Proper Batch Structure	✅	609ms
Batch Format.Flush With No Events Sends Nothing	✅	608ms
Batch Format.Multiple Events Batched Together	✅	614ms
Error Handling.Does Not Retry On 403	✅	2613ms
Error Handling.Does Not Retry On 413	✅	2613ms
Error Handling.Retries On 408	✅	5616ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	610ms
Request Payload.Flags Request Uses V2 Query Param	✅	610ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	609ms
Request Payload.Flags Request Omits Authorization Header	✅	609ms
Request Payload.Token In Flags Body Matches Init	✅	611ms
Request Payload.Groups Round Trip	✅	609ms
Request Payload.Groups Default To Empty Object	✅	610ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	609ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	610ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	609ms
Request Lifecycle.No Flags Request On Init Alone	✅	5ms
Request Lifecycle.No Flags Request On Normal Capture	✅	607ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1214ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	611ms
Retry Behavior.Retries Flags On 502	✅	914ms
Retry Behavior.Retries Flags On 504	✅	911ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1213ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "test: verify SDK timestamps use UTC" | Re-trigger Greptile}

[arnohillen]

(created by claude code)

[arno's agent] Verified at a77ca8c: test-only; the SDK's only emission path is DateTime.utc_now() |> to_iso8601() and the public API accepts no user timestamp, so the offset-conversion requirement has no surface here.