OTHER · ADD/SUBTRACT DAYS
Add/Subtract Days Calculator
Add or subtract any number of days from a date. Enter a negative number of days to subtract. Handles month and year boundaries automatically.
About This Calculator
Find the date N days before or after any starting date. Enter a positive number to add days, a negative number to subtract. Month and year wrapping is handled automatically using UTC calendar arithmetic.
How It Works
The calculator adds the requested number of days to the start date's UTC year, month, and day values. JavaScript's Date.UTC automatically normalises overflowed days (e.g. January 32 → February 1), so wrapping across month and year boundaries is always correct.
The Formula
resultMs = Date.UTC(year, month, day + N), where N is the number of days (positive or negative). The URL uses signedNumberCodec so negative values survive page sharing.
Frequently Asked Questions
- Can I subtract days?
- Yes — enter a negative number in the Days field (e.g. -30 to go back 30 days from the start date).
- Does it handle daylight saving time?
- Yes. The arithmetic operates on UTC calendar dates (year, month, day), not local timestamps, so DST transitions never cause off-by-one errors.