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Date add / subtract calculator

Add or take away years, months, weeks and days from a date, with the month-end clamping made explicit and the weekday the result falls on.

The starting date

How far to move it

Direction

Years and months are applied first, then weeks and days. A date landing on a day that does not exist in the month, the 31st of February, is pulled back to the end of that month.

Resulting date

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Choose a date and how far to move it.

Days are simple. Months are not.

Adding days to a date is exact arithmetic: a day is a day, a week is always seven of them, and leap days are counted as they come. Ninety days after 1 September 2026 is 30 November 2026, and there is nothing to argue about.

Months are different, because they are not all the same length. Adding one month to 1 January moves the date 31 days; adding one to 1 February moves it 28, or 29 in a leap year. The results show the net day count for exactly this reason, so you can see how far the date actually travelled rather than assuming.

The 31st of February problem

What is 31 January plus one month? There is no 31 February, so something has to give. The near-universal convention, and the one used here, is to pull the date back to the last day of the month it lands in: 28 February, or 29 in a leap year.

That has a consequence worth knowing about. Month arithmetic does not reverse. 31 January plus a month is 28 February; 28 February minus a month is 28 January, not the 31st. Nothing has gone wrong. The second calculation cannot know the 28th used to be a 31st. If you need a shift that undoes itself exactly, count in days.

Why the order is fixed

Years and months are applied first, then weeks and days. This matters: starting from 30 January, adding a month and then a day gives 1 March, while adding the day first and then the month gives 28 February. Neither order is wrong and no order avoids every surprise, so this tool commits to one, says which, and shows you where the date stood after the months but before the days.

What it does not do

  • Skip weekends or holidays. Every day is counted. For a deadline in working days, the date difference calculator reports how many weekdays fall in a span.
  • Worry about daylight saving. This counts calendar days, not elapsed hours, so a day on which the clocks move is still one day.
  • Accept a negative amount. The direction is chosen with the toggle, so a minus sign in a field would mean two contradictory things at once.

Questions

What date will it be in 90 days?
Enter the starting date, put 90 in the days field and leave the rest blank. From 1 September 2026 it is 30 November 2026, a Monday. Days are counted exactly, so leap days and month ends are handled without anything special being needed.
What happens when I add a month to the 31st?
The date is pulled back to the last day of the month it lands in. 31 January plus one month is 28 February, or 29 in a leap year, because 31 February does not exist. There is no other sensible answer, but it does mean a month is not always the same number of days.
Why does adding a month and taking it back not return me to the same date?
Because information is lost when the date is pulled back. 31 January plus a month is 28 February; 28 February minus a month is 28 January, not the 31st. Nothing has gone wrong. The calculation cannot know that the 28th used to be a 31st. If you need a shift that reverses exactly, use days rather than months.
In what order are the units applied?
Years and months first, as calendar arithmetic, then weeks and days, as exact day counting. The order changes the answer in some cases, so it is fixed and stated rather than left to chance, and the results show where the date stood after the months but before the days were added.
How many days is a month?
It depends which month. Adding one month to 1 January moves 31 days; to 1 February it moves 28, or 29 in a leap year. The results show the net day count so you can see exactly how far the date actually travelled. If you need a fixed length, use the days field, 30 days is 30 days whatever month it falls in.
Are weeks always seven days?
Yes. A week is seven days with no exceptions, so the weeks field is a convenience rather than a separate kind of arithmetic. Entering 2 weeks and entering 14 days give exactly the same date.
Does this account for daylight saving?
It does not need to. This counts calendar days, not elapsed hours, and a calendar day is a calendar day whether the clocks moved during it or not. Daylight saving only matters when you are measuring a duration in hours, which the time duration calculator does.
Can I work out a deadline that skips weekends?
Not directly. This tool counts every day. For a count of working days between two dates, use the date difference calculator, which reports how many of the days are Monday to Friday. Neither tool deducts public holidays, because they differ by country and by year.

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