Add or subtract years, months, weeks and days from any date and see the resulting date and weekday.
Enter the values to see the result here.
Formula
Days are added on the day-number line, which handles leap years automatically:
Nresult=Nstart±(7w+d)
Years and months are converted to months first, 12y+m, and the month index is moved by that amount. If the target month is shorter than the start day, the day is clamped:
d′=min(dstart,days in target month)
Worked example
Start 30 November 2024, add 3 months and 10 days.
30 November + 3 months = 30 February 2025, which does not exist, so the day clamps to 28 February 2025.
28 February + 10 days = 10 March 2025 (28 + 10 = 38, minus 28 days of February = 10).
Result: Monday 10 March 2025. Adding 100 days directly to 30 November 2024 would give 10 March 2025 as well, but only because of the particular month lengths; that is not generally true.
Common mistakes
Treating 1 month as 30 days. That drifts by up to three days across a year.
Expecting month arithmetic to be reversible: 31 January + 1 month - 1 month is 28 (or 29) January, not 31.
Counting the start day as day 1. This tool adds elapsed days, so adding 1 day to Monday gives Tuesday.
Using it for business deadlines, which need weekends and holidays removed.
Frequently asked questions
What is 1 month after 31 January?
The calculator returns the last day of February (28, or 29 in a leap year). Adding a month never skips into March; it clamps to the end of the shorter month.
In what order are years, months, weeks and days applied?
Years and months first, then weeks and days. This matters at month ends. 31 January plus 1 month plus 1 day is 1 March in a common year, because the month step lands on 28 February first.
Can I add 90 days to find a deadline?
Yes. Put 90 in the Days box. For deadlines that skip weekends, use the working days calculator instead.