Ask a calendar app what “Jan 31 plus 1 month” is, and you’ll often get March 3rd — because JavaScript’s own date math normalizes the invalid “Feb 31” forward. Our new Date Add/Subtract Calculator does the opposite on purpose.
- Clamped month-end math — adding a month or a year resolves the target month first, then caps the day at that month’s real last day. Jan 31, 2026 + 1 month lands on Feb 28, 2026, not Mar 3rd.
- Native rollover where it belongs — days and weeks use the browser’s own date rollover directly, since crossing a month boundary is exactly what’s expected there.
- Leap-year aware — the clamp checks the real day count for the target month and year, so it treats Feb 2028 (29 days) and Feb 2027 (28 days) correctly without any special-casing on your part.
- Add or subtract — one toggle switches direction; days, weeks, months, and years all work identically either way.
- Private by default — the whole calculation runs with JavaScript’s
own
Dateobject in your browser; nothing you enter is sent anywhere.
Pick any start date, choose an amount and a unit, and see the resulting date update instantly — including the exact day of the week.