Days Between Dates

How this is calculated

The count walks the real calendar (leap days included) and reports the difference — the "nights between" convention that deadlines and interest calculations use. Example: 27 February to 1 March 2024 is 3 days, because 2024 has a 29 February. The weekday line subtracts Saturdays and Sundays; public holidays are yours to subtract, since they differ by country.

Inclusive or exclusive: the difference between five days and six

Counting from Monday to Friday gives four if you count the gaps and five if you count the days. Both are correct answers to different questions, and the mismatch causes more disputes than any other date calculation. "How many nights in the hotel?" is exclusive — arrive Monday, leave Friday, four nights. "How many days of leave?" is inclusive — Monday to Friday is five working days off.

This page reports the plain difference between the two dates, which is the exclusive count, and shows the inclusive figure alongside it where it applies. If a contract, a notice period or a leave policy specifies a count, its definition wins over any calculator — and the first question to ask about any deadline expressed in days is whether the starting day counts.

The leap-year rule, in full

A year is a leap year if it is divisible by 4, except that century years are not, except that century years divisible by 400 are. So 1996 and 2024 are leap years, 1900 was not, and 2000 was. The rule exists because the solar year is about 365.2422 days: adding a day every four years overshoots slightly, skipping three century leap days in four hundred years corrects most of the overshoot, and the residual error is around a day in three thousand years.

The most common bug in hand-rolled date arithmetic is remembering only the first clause. It was the specific cause of a spreadsheet-era class of errors around 1900, and it will resurface in 2100, which will not be a leap year despite being divisible by four.

What a calendar calculation cannot know

This calculator counts calendar days. It does not know your country's public holidays, which vary by nation and often by province; nor which of them move each year, as Easter does; nor whether a holiday falling on a Sunday is observed on the following Monday, which differs by jurisdiction. It does not know your employer's shutdown, your contract's definition of a business day, or whether your notice period counts weekends.

Use the raw span from here as the arithmetic foundation, then apply your own local rules on top. For anything with contractual or legal consequence, that second step is not optional.

Historical dates and the eleven missing days

These calculations use the proleptic Gregorian calendar — the modern calendar projected backwards — for every date, including those before it was adopted. That is the standard computational convention, but it means results for dates before adoption will not match a contemporary document. Britain and its colonies switched in September 1752, and 3–13 September that year simply did not occur locally; other countries changed between 1582 and 1927. For genealogy or archival work, note which calendar a source used before subtracting anything.