How days between two dates are counted
Counting days sounds trivial until the months have different lengths, a leap year turns up and a clock change gets in the way. The reliable method turns each date into an absolute day count from a fixed reference and subtracts. That is exactly what this calculator does, which is why it makes no difference how many 31-day months lie in between.
Does the first day count?
This is where most of the confusion comes from, and the answer depends on what you need it for.
- Difference between dates (the usual case): the starting day does not count. From 1 to 31 January there are 30 days.
- Length of a stay or an event: both ends count. From 1 to 31 January is 31 nights of hotel.
For deadlines under most administrative rules the clock starts the day after notice is given. A 10-day term from Monday the 3rd runs out on Thursday the 13th, not Wednesday the 12th. Which rule applies is set by the procedure you are under, so it is worth checking rather than assuming.
Calendar days, business days and working days
| Kind | What it includes | Where you meet it |
|---|---|---|
| Calendar days | Every day on the calendar | Contracts, rentals, warranties |
| Business days | No Saturdays, Sundays or public holidays | Administrative and court deadlines |
| Working days | Whatever the contract says; sometimes includes Saturday | Annual leave and shift patterns |
The difference is not small: 30 calendar days is a month; 30 business days is six weeks. When a contract or a notice says “days” and nothing else, it is worth finding out which of the three it means.
How to count them by hand
For two dates in the same year, the quickest method is the day-of-year one:
- Add up the days of the whole months before each date. January 31, February 28 (29 in a leap year), March 31, April 30, May 31, June 30, July 31, August 31, September 30, October 31, November 30.
- Add the day of the month. That gives you the day of the year for each date.
- Subtract the two numbers.
For example, 15 March to 20 July 2026. March 15 → 31 + 28 + 15 = day 74. July 20 → 31 + 28 + 31 + 30 + 31 + 30 + 20 = day 201. Difference: 201 − 74 = 127 days.
If the dates fall in different years, count the days left to the end of the first year, add 365 for each whole year in between (366 for a leap year) and add the day of the year of the final date.
Leap years and clock changes
A year is a leap year if it divides by 4, unless it divides by 100 and not by 400. That is why 2024 was one, 1900 was not and 2000 was. Every leap year adds 29 February, and forgetting it is the most common mistake when counting long periods by hand.
The clock change causes a subtler failure. In countries that use daylight saving there is one night of 23 hours and one of 25. If the calculation subtracts local timestamps and divides by 24 hours, the answer shifts by a day. This calculator works in universal time, where every day lasts exactly the same.