Date Difference Calculator — Days, Weeks, Years Between Dates

"Months between two dates" has no single correct answer

Days are a fixed length and months are not, which is the root of everything on this page. From 31 January to 28 February is 28 days. From 31 March to 30 April is 30 days. Both are commonly called "one month".

Two identical month counts spanning different numbers of days is not a bug in any calculator — it is a property of the calendar. It is also why date libraries disagree with one another and with spreadsheets, and why the disagreement is not resolvable by picking a better tool.

The end-of-month problem

Adding one month to 31 January has no correct answer, because 31 February does not exist. This tool clamps to the last valid day of the target month, giving 28 or 29 February depending on the year.

Other implementations roll forward into March instead, and both choices are defensible. The practical consequence is that a date calculation which looked right in testing can be wrong in a short month or a leap year — precisely the cases least likely to be in your test data.

This tool counts elapsed time, not calendar days

From 1 March to 3 March it reports 2 days, not 3, because it measures the interval between the two dates rather than the days the range touches. This is exclusive counting.

Both conventions are legitimate and each is standard somewhere: a hotel stay from 1 to 3 March occupies three calendar days and is billed as two nights, and age is counted the same way this tool counts. If you need the number of days your event touches, add one. Knowing which convention you are being given is the whole point.

Leap years shift any span that crosses one

The rule in full: a year is a leap year if divisible by 4, except centuries, unless divisible by 400. So 1900 was not a leap year and 2000 was — the exception people usually forget, and the reason some date code failed in 2000.

Any span crossing 29 February is a day longer than the same span in an ordinary year. From 28 February to 1 March is 1 day normally and 2 days in a leap year. That single day is enough to move an anniversary, a deadline or a billing date.

What the tool reports

Six figures: a combined years-months-days breakdown, plus total days, total weeks, total hours and total minutes. Everything derives from the elapsed milliseconds between the two dates.

Note that weeks are the whole number of complete weeks — 20 days reports as 2 weeks, not 3 — so the weeks figure discards the remainder rather than rounding it.

The practical advice this all points to: for anything contractual, count in days. Days are unambiguous and months are not. If a period genuinely must be expressed in months, define the end-of-month behaviour in writing rather than leaving it to whichever tool each side happens to use. This is general information, not legal advice.

Measuring a gap between two dates and counting somebody's age look like the same problem and are not — age counts calendar units from a fixed anniversary. The age calculator does that properly.