Date Add / Subtract Calculator
Add or subtract days, weeks, months or years from any date.
What the Date Add / Subtract Calculator does
Adding calendar periods is not simple arithmetic because months vary in length. This calculator applies years and months first, clamps the day of the month if the target month is shorter, then applies weeks and days.
Formula
Apply years, then months, clamping the day to the last valid day of the target monthThen add or subtract weeks × 7 + days
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Start date | date | Yes | — |
| Operation | one of 2 options | Yes | — |
| Years | number | Optional | Accepts -9999 or more, up to 9999. |
| Months | number | Optional | Accepts -120000 or more, up to 120000. |
| Weeks | number | Optional | Accepts -520000 or more, up to 520000. |
| Days | number | Optional | Accepts -3650000 or more, up to 3650000. |
How to use it
- Choose Operation.
- Enter Start date.
- Optionally add Years, Months and Weeks.
- Select Calculate.
Worked example
Add 3 months and 10 days to 31 January 2026.
- Start
- 2026-01-31
- Operation
- Add
- Months
- 3
- Days
- 10
31 January + 3 months = 30 April (clamped), then + 10 days = 10 May 2026.
Frequently asked questions
Why does adding one month to 31 January not give 31 February?
Because that date does not exist. The convention is to clamp to the last day of the target month, giving 28 or 29 February.
Does adding 30 days equal adding one month?
Rarely. Only April, June, September and November have 30 days. Contracts usually specify one or the other deliberately.