Date Difference Calculator

Inclusive or exclusive

The first question any date difference raises is whether both endpoints count. From 1 March to 3 March is two days if you count the gap and three if you count the days involved. Contracts, notice periods and hire durations differ on this, and the convention is rarely stated. Decide which you need before trusting any figure: an off-by-one here has ended up in court more than once.

Months are not a fixed unit

A difference in months is ambiguous whenever the day-of-month does not exist in the target. What is one month after 31 January? Different systems answer 28 February, 2 March or 3 March. The common convention, used here, clamps to the last valid day of the target month, so 31 January plus one month is 28 February. Expressing long spans in days avoids the ambiguity entirely.

Working days need a definition

Excluding weekends is straightforward. Public holidays are not: they vary by country, by region within a country, and by year, and some move with Easter. A working-day count is only as good as the holiday list behind it. For anything contractual, confirm which calendar applies rather than assuming the default.

Leap years and the 400-year rule

Every year divisible by 4 is a leap year, except centuries, except centuries divisible by 400. So 1900 was not a leap year and 2000 was. Spreadsheets have historically shipped with a deliberate bug treating 1900 as a leap year, retained for backward compatibility, which makes date arithmetic spanning that year off by one in some tools.

Time zones make a day ambiguous

Two timestamps can fall on different calendar dates depending on the zone used to render them, so a difference in days can vary by one. Where a date difference matters legally or financially, fix the time zone explicitly rather than relying on the viewer's local setting.