[jcd1991]

Transport timestamp headers currently use yyyy-MM-dd HH:mm:ss:ffffff Z, which stock date parsers cannot read. This completes the writer portion of #3645: both timestamp writers now emit invariant round-trip ("o") UTC values, preserving all seven fractional-second digits. Readers continue accepting legacy headers. No schema changes.

The regression covers scheduling, expiration and sent timestamps, non-UTC input, three cultures, null headers, and legacy timestamps through binary persistence. Migration guidance explains receiver-first rollout.

Release sequencing — draft

This must not ship to mixed fleets until receivers can read the new format. The 6.x reader change merged in #3656; the 5.x backport merged in #4583. NuGet's latest 5.x package at verification was 5.41.0, and its tagged source still uses legacy-only readers. The backport needs a published release and a documented receiver version floor before this writer change is released to those fleets. Maintainer confirmation of release sequencing is needed before marking this ready.

Verification

• Timestamp regression: 12 passed on .NET 9 and .NET 10. With the original writers restored, the three culture/precision cases fail (9 pass, 3 fail).
• Envelope-related CoreTests: 207 passed on each target.
• Full CoreTests on each target: 4,031 passed, 8 skipped, 1 failed out of 4,040. These are not clean full-suite results.
  • .NET 10: ignoring_traffic_for_every_session_on_a_host.without_the_rule_the_same_traffic_holds_the_session_open; expected timeout, observed completion. Its entire class passed 9/9 in an isolated rerun.
  • .NET 9: exception_event_on_failed_attempt_span_5311.each_attempt_of_a_retried_then_dead_lettered_message_records_its_exception; expected two spans, observed zero. It also fails with the original timestamp writers restored.
• Full solution Release build for net9.0 attempted: blocked by Grpc.Tools' bundled macOS x64 protoc failing on this ARM Mac (Bad CPU type in executable). CoreTests builds succeeded on both targets.
• Scoped style and analyzer checks passed with --verify-no-changes. The test style check excludes IDE1006 because this existing fixture uses the repository's snake_case test naming; production style checks include naming.
• git diff --check passed. Final review found no additional implementation issues.