Finds the date a given number of days after — or, with a negative number, before — a starting date. Leap years are handled correctly.
This tool finds the date a given number of days after, or before, a starting date. Payment terms, delivery dates and countdowns to an anniversary all come down to this one calculation.
Here is the day number of the starting date, the days to add and the day number of the answer. The date is turned into a day number, the offset is added, and the result is turned back into a date. Month ends and leap years need no special treatment: the addition absorbs them.
A negative number of days moves backwards instead. To find the date 100 days earlier, enter −100.
Adding 100 days to 27 July 2026 gives 4 November 2026, a Wednesday.
Four days remain in July, then August has 31, September 30 and October 31, which comes to 96. The remaining four days land on 4 November. The weekday advances by the remainder of 100 divided by 7, which is 2, so Monday becomes Wednesday.
Adding whole months is a different problem and this tool will not do it. A month runs from 28 to 31 days, so days and months cannot be mixed in one sum.
The weekday is given by its English name. The weekday number beside it follows ISO 8601, where Monday is 1 and Sunday is 7.