Days between two dates

Counting the last day, or not

This is the question behind most disputed date sums, and there is no universally right answer, so it is a switch. Left off, 1 January to 2 January is one day: the ordinary reading of “how many days between”, and what every date library does.

Switched on it is two. That is what contract and statutory wording means by “clear days”, “inclusive” or “both dates”, what a holiday booking means when it counts days rather than nights, and what a hire company charges you for.

The two differ by exactly one, which sounds trivial and is the amount that matters when a deadline is tight. Check which convention your wording uses before relying on either.

Times, and the two days a year that are not 24 hours

Tick include times of day and the answer becomes an exact span: days, hours and minutes, plus the total in hours, minutes and seconds. The last-day switch disappears at that point, because counting the last day is a whole-day idea and exact times already give an exact span.

The part worth knowing is what happens across a clock change. Midnight to midnight on 29 March 2026 is 23 hours, not 24: the clocks go forward at one in the morning and that hour never happens. Midnight to midnight on 25 October is 25 hours, because the hour between one and two runs twice.

This measures real elapsed time rather than doing wall-clock arithmetic, so it gets both right, and it tells you when a change fell inside the span rather than leaving you to wonder why the figure looks odd. If you are counting a shift, a flight or anything billed by the hour, that hour is real money.

Months are counted, not divided

A month is 28, 29, 30 or 31 days depending on which one it is, so there is no number of days you can divide by to get months. Plenty of calculators divide by 30, or by 30.44 to be clever about it, and produce a figure that is wrong for most pairs of dates while looking precise.

This walks the calendar instead, counting only complete months. From 31 January to 28 February it reports 0 months and 28 days, because the 31st has not come round again. Some tools clamp and call it a whole month. Neither is wrong; they answer slightly different questions. This one never reports a month that has not finished, which is the reading a contract takes.

Adding months is not symmetric

Move a date forward by months or years and it lands on the same day number, unless that day does not exist in the target month. One month after 31 January is 28 February, or the 29th in a leap year, because February has no 31st. Every tool that does this properly clamps the same way.

The consequence is worth knowing: going forward and back does not always return you. A month forward from 31 January is 28 February, and a month back from there is 28 January. Nothing can fix that. The calendar is simply like this, and a tool that hid it would be lying about the arithmetic rather than solving it.

Leap years, exactly

The count runs over real calendar dates, so a span that crosses 29 February picks up that day and one that does not, does not. No average year length is involved anywhere.

The rule, for the record: a year divisible by 4 is a leap year, unless it is divisible by 100, unless it is also divisible by 400. So 2000 was a leap year and 1900 was not, and 2100 will not be.

Weekdays are not working days

The weekday figure here counts Monday to Friday and nothing else. It knows nothing about bank holidays, so it is the right number for “how many weekdays” and the wrong one for anything with a legal deadline attached.

For notice periods, filing deadlines, payment terms and consultation windows, use the working days calculator, which excludes bank holidays using the official GOV.UK list and tells you which ones it removed.