OKF timestamps: support datetime+offset (upstream SPEC amendment still labeled 0.2) #41

Summary

Upstream OKF SPEC (still Version 0.2) was amended so every timestamp-valued key is an ISO 8601 datetime with an explicit UTC offset, not a bare YYYY-MM-DD date.

Upstream commit: GoogleCloudPlatform/knowledge-catalog@62432a0
Date (UTC): 2026-08-21T01:44:53Z
Same content on open-knowledge-format: https://github.com/GoogleCloudPlatform/open-knowledge-format/blob/main/SPEC.md

Already synced

src/Google/SPEC.md already matches upstream (dotnet-file sync in 9236ea1).

Still behind

• specs/okf-0.2.md — still documents date-only stale_after, sources[].last_modified, and usage_window
• schemas/okf-0.2.json — staleAfter, lastModified, and usageWindow.from/to still format: "date"
• Tool behavior likely still date-day based (TrustSignals, GraphBuilder, BundleChecker, samples, tests)

Impact on the tool

Upstream now defines:

• stale_after as an absolute instant; stale when now >= stale_after
• last_modified / usage_window as datetimes with offsets
• Examples use forms like 2026-09-23T00:00:00Z

If the tool continues to parse/compare these as calendar dates only, graphs and checks will disagree with the live SPEC (and with src/Google/SPEC.md).

Requested work (do not treat as assigned yet)

1. Align parsers / staleness checks with datetime+offset semantics (reject or ignore bare dates if that matches SPEC intent).
2. Update schemas/okf-0.2.json formats (date → date-time) for the affected fields.
3. Refresh samples/tests that still emit date-only values.
4. Cross-link the companion spec-snapshot PR that refreshes specs/okf-0.2.md (opening separately).

Notes

• No version bump upstream (still 0.2); this is an in-place SPEC amendment, not OKF 0.3.
• Spec-only snapshot vendoring is tracked separately from this tool/schema work.