Chat task creation and updates now reject impossible calendar dates in timestamps with Z or an explicit UTC offset. Previously 2026-02-30T12:00Z silently became March 2 before being stored. The parser applies its existing calendar-date validation before converting these timestamps; valid leap dates, offset crossings, fractional seconds, years 0000–0099 and 24:00 retain their existing behavior.

Added native tests that verify rejected dates neither create a task nor change an existing task's due date, plus valid timestamp controls. A paired replay of the actual parser and task tool executor with locked Zod 4.6.5 reproduces five invalid parser dates and ten invalid create/update writes on base 8a5d8a84; the fix rejects all of them before writing. README task documentation describes the validation.

Validation on source ceabeede283a1c3d1c719589228a6e952bc1002c: full lint, typecheck and build passed; all 705 native tests passed (including 11 new date tests in workerd), and all 27 local Playwright E2E tests passed. Qualification run and logs. The workflow helper is 76db44585d3629a728a457117df1653383b59981; its only change is the validation workflow, which checks out and asserts the source SHA, tree and parent. Tests use mock/local services without account credentials; no real-account smoke test was run. Installer packing was omitted because the installer is unchanged.