fix: [SDK-5284] parse IAM end_time as UTC instead of device timezone

[fadi-george]

Description

One Line Summary

Parse and format ISO-8601 Z timestamps in UTC so scheduled In-App Messages stop expiring early (or late) on devices outside UTC.

Details

Motivation

Fixes SDK-5284. A customer in JST (UTC+9) reported that In-App Messages with a schedule end time never displayed on Android, while iOS displayed them correctly.

The server sends IAM end_time as ISO-8601 with a literal Z suffix, for example 2026-09-18T21:30:00.000Z. DateUtils.iso8601Format() used the pattern yyyy-MM-dd'T'HH:mm:ss.SSS'Z'. The quoted 'Z' is a literal character match, not a timezone specifier, so SimpleDateFormat parsed the wall-clock portion in the device's default timezone. On a JST device 21:30Z became 21:30 JST (12:30Z), nine hours early, so InAppMessage.isFinished returned true and the message was skipped.

iOS sets its formatter to UTC and uses XXXXX, which is why the same IAM displayed there.

Scope

• DateUtils.iso8601Format() now sets the formatter timezone to UTC. The pattern and wire format are unchanged.
• Affects InAppMessage end time parsing and end_time formatting in toJSONObject().
• CustomEventBackendService already applied UTC itself, so its payload is unchanged.
• No public API changes.

Testing

Unit testing

Added DateUtilsTest covering both directions under an Asia/Tokyo default timezone:

• parsing 2026-09-14T11:30:00.000Z yields the exact UTC instant
• formatting that instant yields 2026-09-14T11:30:00.000Z

The parse test failed before the fix with a 9 hour offset and passes after. IAM module unit tests pass.

Manual testing

Switched my Mac and Android emulator timezones to Tokyo (UTC+9) and scheduled an IAM to stop showing about 10 minutes in the future. Validated that the IAM displays on both Android and iOS with this branch.

I also validated the original issue on main: with the same setup, iOS displayed the IAM but Android skipped it. Logs on main showed the server end_time of 2026-09-18T21:30:00.000Z being parsed as Fri Sep 18 21:30:00 GMT+09:00 2026 (already in the past). On this branch the same value parses as Sat Sep 19 06:30:00 GMT+09:00 2026 and the IAM displays.

Environment: Android emulator (Pixel, Android 16 / API 36), OneSignal example app, iOS Simulator.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

[github-actions]

📊 Diff Coverage Report

Diff Coverage Report (Changed Lines Only)

Gate: aggregate coverage on changed executable lines must be ≥ 80% (JaCoCo line data for lines touched in the diff).

Changed Files Coverage

• DateUtils.kt: 3/3 touched executable lines (100.0%) (4 touched lines in diff)

Overall (aggregate gate)

3/3 touched executable lines covered (100.0% — requires ≥ 80%)

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