requireStartDateGranularity (store/actions.ts) and its import-path twin
(export/json.ts) checked date SHAPE with a regex, not date VALIDITY, so
"2026-09-31" (September has 30 days) passed both and threw later out of
foldBalances instead of at the write boundary. isValidStartDate
(RecurringSection.tsx) had the same hole for a day-shaped value under
everyNMonths. Separately, monthOf's regex (domain/months.ts) was
unanchored at the end, so "2026-091" was silently read as September 2026.
Route day-shaped values through toDayOrdinal and month-shaped values
through monthOf (now anchored to accept only "YYYY-MM" or "YYYY-MM-DD" in
full, never a prefix) at all three write boundaries, converting whatever
they throw into each file's own error style.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Shape-valid but out-of-range months (e.g., month 13 or 00) previously
normalized silently through ordinal arithmetic, resulting in wrong months
downstream. Now throw on invalid month values (< 1 or > 12), with the
offending value in the error message.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
