Date Calculator (Add/Subtract)

About this calculator

This calculator adds or subtracts a chosen number of days, months and years from a starting date and returns both the resulting calendar date and its weekday, correctly handling month-length differences and leap years along the way (adding a month to January 31 doesn't simply land on February 31, which doesn't exist). Manual date arithmetic is deceptively error-prone once leap years and varying month lengths are involved.

It's used for figuring out contract or warranty end dates, project deadlines a fixed number of working days or months out, an event date a certain number of weeks before or after another, or simply what weekday a future or past date fell on. Anyone planning around a fixed offset from a known date — rather than a fixed target date — reaches for this instead of counting on a calendar by hand.