Date Add / Subtract
Add or subtract years / months / days from a base date, with month-end and year-end handled automatically (e.g. Feb 28 plus 1 day becomes Mar 1).
Day付の加算／減算
Given a base date, enter the years, months and days to add or subtract and the tool computes the target date and its timestamp. It handles month-end and year-end correctly: for example Feb 28 plus 1 day becomes Mar 1, and Dec 31 plus 1 year is still Dec 31.
All computation happens in your browser.
Features
- Years/months/days separately — Three independent fields, any combination.
- Month-end clamp — E.g. Jan 31 plus 1 month becomes Feb 28/29, not an overflow into March.
- With timestamp — The result also gives the matching Unix seconds timestamp.
How to use
- Pick a base date.
- Enter the years / months / days to add or subtract (may be negative).
- The result shows the target date and timestamp instantly.
FAQ
Why is "Jan 31 plus 1 month" Feb 28, not Mar 3?
February has no 31st, so adding a month directly would "overflow" into early March, which feels wrong in most cases. This tool clamps the day to the last day of the target month, giving Feb 28 (or 29 in a leap year).