Add or Subtract Days from a Date
Find the date a number of days, weeks, months or years from any date.
Good to know. Ninety days and three months are rarely the same thing. Months don’t agree on their length.
How it works
In plain words
Days and weeks are added on a continuous calendar. Months and years move the calendar month and keep the same day, or use the last day of the month if that day doesn’t exist.
The formula
date ± days date ± (weeks × 7) days date ± months (day clamped to month length)
Worked example
90 days after 15 March 2026 is Saturday, June 13, 2026.
Sources
- ISO 8601, Date and time format Calendar dates in the Gregorian calendar, and weeks that start on Monday.
Sources checked September 29, 2026. 9 automated tests check its answers on every change.
About the add or subtract days from a date
Deadlines, notice periods, return windows, visas and medication schedules are often counted in days: "within 30 days", "90 days from arrival". This calculator gives you the exact date, including the day of the week, so you can see straight away if it lands on a weekend.
Days and weeks are straightforward: the calculator steps along the real calendar, so month lengths and leap years are always right.
Months and years need a rule, because months have different lengths. Adding one month to 15 March gives 15 April, but adding one month to 31 January can’t give 31 February. In that case we use the last day of the month, 28 February (or 29 in a leap year). This is the same rule spreadsheets use in their EDATE function and most legal and banking conventions follow, but check your contract if it matters, since some count "a month" as 30 days.
Counting from today? The start date defaults to today in your own time zone. Remember to check whether the day you start from counts as "day zero" or "day one" in your situation. Here, adding 1 day to Monday gives Tuesday.
Questions people ask
What date is 90 days from today?
Enter today’s date (the default), choose Add, 90, Days, and the calculator shows the date and weekday.
What happens when I add a month to 31 January?
You get the last day of February, because February has no 31st. This is the usual rule, used by spreadsheets too.
Does day 1 count as the start date?
No. Adding 1 day to Monday gives Tuesday. If your rule counts the start date as day 1, subtract one day from the result.