Date Calculator: Add or Subtract Time

How the new date is worked out

Years and months are added first, on the calendar: adding 1 month to 10 March gives 10 April, whatever the number of days in March. Weeks and days are then added as exact numbers of days (a week is 7 days). This is the order most people expect, and it matches how contracts and due dates are usually described.

Example: 90 days after Saturday 26 September 2026 is Friday 25 December 2026. Eighteen months before 26 September 2026 is Wednesday 26 March 2025.

Month ends and leap years

When the starting day does not exist in the resulting month, the result moves back to the last day of that month, and the calculator tells you it has done so.

Because of this, adding 1 month and then subtracting 1 month does not always return the original date: 31 January + 1 month = 28 February, and 28 February − 1 month = 28 January. When an exact number of days matters (for example a 30-day return period), enter days rather than months.

Common questions

Is the start date counted as day 0 or day 1?

Day 0. “1 day after” Monday is Tuesday. If your rule says the start date is day 1 (some notice periods work this way), enter one day fewer.

Does it skip weekends and holidays?

No, every calendar day is counted. To count working days between two dates, use the days between dates calculator, which lets you choose your weekend days.

How far back or forward can I go?

From the year 1 to the year 9999, using the Gregorian calendar throughout (the proleptic Gregorian calendar used by ISO 8601). Historical dates before a country adopted the Gregorian calendar (1582 onward, as late as the 1920s in some countries) may differ from dates written in records of the time.