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Date calculator
Add days, weeks, or months to a date, or count the time between two dates.
One zone owns the arithmetic, no silent machine default.
- One zone owns the arithmetic. A naive input (2026-03-07 03:00) is wall time IN the selected zone; an input with an offset (…Z or …-05:00) is that exact instant, re-expressed in the zone. Both rules are stated, never guessed.
- Calendar fields are wall-clock operations, clock fields are elapsed: 2026-03-07 03:00 New York + 1 day = 03:00 next day (23 real hours across spring-forward); + 24 hours = 04:00. Both facts are the engine's, test-pinned.
- Month ends clamp, they never roll over: 2026-01-31 + 1 month = 2026-02-28, and the note says the clamp fired.
- Diffs state two answers on purpose: exact elapsed time and the calendar decomposition. Across a DST transition they legitimately disagree. The page shows both, labeled.
- Every field is a bounded whole number (years ±1,000, days ±100,000, …) so a typo cannot ask for a nonsensical date; impossible dates (February 30) are refused.
Done
Finished here in the tab. Nothing was uploaded.
engine: date-fns over @date-fns/tz: the zone is part of the computation, not the display