Counting the days between two dates
The calculator gives every date a serial number: the count of days since a fixed reference day (January 1, 1970). Subtracting the start date’s number from the end date’s number gives the gap in whole days. Because it works with calendar dates rather than clock hours, a 23-hour or 25-hour day around a daylight saving change still counts as exactly one day, and your time zone cannot shift the answer.
The plain difference answers the question “how many days from one date to the other”. It counts one of the two boundary days, not both. That is the right number for nights in a hotel, the age of a parcel in transit or the days until an event. Ticking Include the end date adds one, which is what you want when every listed day is used: a festival that runs from the 2nd to the 6th lasts five days, not four.
This is the fencepost problem. A fence four sections long needs five posts, and a span four days long touches five dates:
The other results are rearrangements of the same span:
- Weeks and days splits the total into whole weeks and the days left over.
- Years, months and days counts whole calendar months first, then the remaining days. A month is measured from a day number to the same day number in the next month, so its length depends on which month it is. With the box ticked, this breakdown also includes the extra day.
- Weekdays counts the Monday-to-Friday dates among the days counted: from the start date up to the day before the end date, or up to and including the end date when the box is ticked. Public holidays are not removed.
Formulas and the leap year rule
The day count is exact for any pair of dates on the Gregorian calendar. Its leap year cycle repeats every 400 years, which hold 146,097 days, an average year of 365.2425 days.
The leap year rule is the one part of the count that people often get wrong by hand. February has 29 days in a leap year and 28 otherwise:
| Year | Divisible by 4? | Century year? | Leap year |
|---|---|---|---|
| 1900 | Yes | Yes, not divisible by 400 | No |
| 2000 | Yes | Yes, divisible by 400 | Yes |
| 2024 | Yes | No | Yes |
| 2026 | No | No | No |
| 2028 | Yes | No | Yes |
| 2100 | Yes | Yes, not divisible by 400 | No |
Months are a second trap. They run from 28 to 31 days, so “two months” can be anything from 59 days (January and February in a common year) to 62 days (July and August). That is why the calculator reports months and days separately instead of dividing the total by 30.
Worked examples
- Days left in October after the 13th: 31 − 13 = 18.
- Add all of November (30) and the first 25 days of December: 18 + 30 + 25 = 73 days.
- In weeks: 73 ÷ 7 = 10 remainder 3, so 10 weeks 3 days.
- In months: October 13 to December 13 is 2 months, and December 13 to 25 is 12 days.
- Without the end date: 6 − 2 = 4 days.
- The course uses every date in the range, so tick the box: 4 + 1 = 5 days.
- All five dates are Monday to Friday, so the weekday count is also 5. Leave the box unticked and both figures drop to 4, because Friday the 6th is no longer counted.
- 2024 is divisible by 4 and is not a century year, so February 2024 had 29 days.
- 2025 is not divisible by 4, so February 2025 had 28 days.
- In both years, February 1 to March 1 is one calendar month and 0 days, even though the day counts differ.
Ways a day count goes wrong
- Adding one by reflex. “From the 10th to the 14th” is 4 nights in a hotel but 5 days of a conference. Decide whether the last date is used before ticking the box.
- Assuming 30-day months or 365-day years. January 1 to December 31, 2026 is 364 days apart, or 365 with the end date included. A year that contains February 29 adds one more.
- Forgetting the century exception. 1900 had no February 29 and 2100 will not have one either. A rule of “every fourth year” is right for 2001 to 2099 only.
- Reading weekdays as working days. The weekday count keeps public holidays. Use the business days calculator to list and skip them.
- Misreading a typed date. 03/04/2026 is March 4 in the United States and 3 April in most other countries. The date pickers on this page avoid that, and writing dates as 2026-03-04 (year, month, day) does too.
The calculator uses the Gregorian calendar for every date, including dates before a country adopted it. Britain and its colonies switched in 1752, for example, so day counts that reach back past a local switch will not match the dates written in records of the time.
Questions people ask
How many days are in a year?
365 in a common year and 366 in a leap year. Over the full 400-year Gregorian cycle the average is 365.2425 days.
Is the start date counted?
The plain difference counts the gap between the dates, which treats one boundary as day zero. Ticking “Include the end date” counts every date from the start to the end, both included.
How many weeks are in a year?
52 weeks and 1 day in a common year, and 52 weeks and 2 days in a leap year. Those spare days are why a date moves one weekday later each year, or two after a February 29.
Sources
- Which years are leap years and can you have leap seconds?, Royal Museums Greenwich.
- RFC 3339: Date and Time on the Internet: Timestamps (definitions and Appendix C, Leap Years), Internet Engineering Task Force.