How are months added when the day does not exist?

Month math clamps to the last valid day. Adding one month to January 31 lands on February 28 (or 29 in a leap year) rather than spilling into March, which matches how most calendar libraries behave.

In what order are years, months, and days applied?

Years and months are applied first to shift the calendar, with day-of-month clamping, and then the day count is applied as exact 24-hour increments on top of that result.

Does this handle leap years?

Yes. Because the date math uses real calendar arithmetic, February 29 is produced only in leap years, and adding a year to a leap day clamps to February 28 in common years.

Why does the tool use UTC internally?

All math is done in UTC so daylight-saving-time transitions can never shift the calendar date by a day. The output is a pure calendar date with no timezone offset.

Can I subtract a duration to find a past date?

Yes. Choose Subtract and enter the amounts to count backward. Negative numbers in the fields are treated as their absolute value so the chosen operation always controls the direction.

Date Add/Subtract Duration

Add or subtract a duration from any date

This calculator shifts a date forward or backward by a combination of years, months, and days, and tells you the exact resulting calendar date and weekday. Use it for deadlines, contract end dates, subscription renewals, age milestones, or any “X days/months from now” question.

How it works

The calculation is done in two stages using UTC dates, which avoids daylight-saving shifts moving the result by a day. First the year and month components are applied: the requested years and months are added to (or subtracted from) the start date, and any month overflow is normalized into years. Crucially, the day of the month is then clamped to the last valid day of the target month — so Jan 31 + 1 month becomes Feb 28 (or Feb 29 in a leap year) rather than rolling into March.

Second, the day count is applied as exact 24-hour increments on top of that clamped date. Keeping months and days separate is what makes the result match human intuition: “one month and ten days from the 31st” behaves the way a calendar app would, not as a raw addition of approximate day counts.

Tips and example

Starting from 2026-01-31 and adding 1 month yields 2026-02-28, because February has no 31st. Add 0 years, 1 month, 1 day to that same start and you get 2026-03-01. The resulting weekday is shown beneath the date, which is handy when you need a deadline to land on a business day. Negative entries are interpreted by magnitude, so the Add/Subtract selector always determines the direction of travel.