Date Calculator
Find the number of days between two dates, or add and subtract days, weeks, months, and years from a date to get a new date.
How it works
How dates are calculated
Difference = end date − start date (in days) Add: shift years and months on the calendar, then add days
Date math uses the real calendar, so leap years and different month lengths are handled correctly. When adding years and months, the day of the month is kept where possible (clamped to the last day when the target month is shorter); the day offset is then applied as a fixed number of days.
Worked example
From January 1, 2026 to December 31, 2026 is 364 days (11 months and 30 days). Adding 90 days to January 1, 2026 lands on April 1, 2026, a Wednesday.
Frequently asked questions
Does the calculator count both the start and end date?
It counts the number of nights between the two dates — the difference in calendar days. If you need to include both endpoints (for example, counting event days), add one to the result.
How are leap years handled?
Fully. The calculation uses actual calendar dates, so February 29 and the varying lengths of months are all accounted for automatically.
What happens when I add a month to January 31?
The day is clamped to the last valid day of the target month, so January 31 plus one month becomes February 28 (or 29 in a leap year).