Start date, Operation, Amount, Unit
Add or Subtract Days Calculator
Find the date a number of days, weeks, months or years before or after another date.
Waiting for input
How this calculator works
Days and weeks are added as an exact number of days. Months and years are added on the calendar: if the target month is shorter, the result is the last day of that month, so one month after January 31 is February 28 (or 29 in a leap year).
Formula
- Result = start date ± amount
- 1 week = 7 days
- Month and year steps keep the day of the month, clamped to the month's last day
Worked example
90 days after January 1, 2026
- Start date
- 2026-01-01
- Operation
- Add
- Amount
- 90
- Unit
- Days
ResultWednesday, April 1, 2026
90 days after Thursday, January 1, 2026.
Frequently asked questions
What is 90 days from today?
Leave the start date as today, choose Add, enter 90 and select Days. The result shows the date and its weekday.
What happens when I add a month to the 31st?
If the next month has fewer days, the result is that month's last day. January 31 plus one month is February 28, or February 29 in a leap year.
Is the start date counted as day one?
No. Adding 1 day to Monday gives Tuesday. If your deadline counts the start date as day one, enter one day less.
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Results are estimates based on the values you enter and the published method shown above. They do not replace professional medical, financial or legal advice. Method reviewed October 7, 2026. About this site