How to use
Start by choosing one of the two options under “Calculation” at the top.
Between two dates works out how many days, months and years lie between the “Start date” and the “End date”. Use it when you need the length of a period, such as how long you have been in a job, how long a project ran or a contract term. The end date starts out as today’s date on the device you are using, so picking a start date is enough to get the period up to today. If you enter an end date that is earlier than the start date, the calculator swaps the two and tells you so below the result: “The end date was before the start date, so the two were swapped.”
Add or subtract finds the date that falls a number of days, weeks, months or years after or before the “Reference date”. Use it when you need a particular date, such as the day a contract expires or a submission deadline. Enter a number in “Amount” (it starts at 100), then choose days, weeks, months or years under “Unit” and After or Before under “Direction”. The “Resulting date” appears with its weekday, and below it the distance is also given in days, as in “100 days after the reference date”.
Reading the results
Between two dates, the same period is shown in four ways. Taking March 2, 2024 to September 28, 2026 as an example:
| Item | Result | Meaning |
|---|---|---|
| Total days | 940 days | The difference between the two dates, counted in days |
| In weeks | 134 weeks 2 days | The total days grouped into weeks of 7 |
| In months | 30 months 26 days | Whole months counted on the calendar, plus the days left over |
| In years and months | 2 years 6 months 26 days | The months figure grouped into years |
Total days is the end date minus the start date, so the start date itself is not included. If you worked from September 1 to September 30, you worked 30 days, but total days comes out as 29. When both the first and the last day should count like this, turn on “Count the start date as a day (e.g. Sep 1–30 = 30 days)”.
The option adds one day to total days and to the weeks figure only. In the example above they become 941 days and 134 weeks 3 days. The months figure and the years-and-months figure are counted by calendar date, so they stay the same. For the same reason, January 1 to December 31, 2026 is 364 days with the option off and 365 days with it on, and 11 months 30 days either way.
Months are counted on the calendar
Months are counted by calendar date, not in blocks of 30 days. March 15 to September 15 is exactly 6 months, and one day short of that, to September 14, is 5 months 30 days. If the month where the period ends has no matching day, the month is complete on that month’s last day. January 31, 2026 to February 28 is 1 month 0 days, and to March 1 it is 1 month 1 day.
A year is not a block of 365 days either. January 1, 2026 to January 1, 2027 is 365 days, and January 1, 2024 to January 1, 2025, which contains a leap day, is 366 days, yet both come out as 1 year. Leap years follow the Gregorian rule: a year divisible by 4 is a leap year, except that a year divisible by 100 is not, unless it is also divisible by 400.
Add or subtract examples
Results when the reference date is September 28, 2026:
| Calculation | Resulting date |
|---|---|
| 100 days after | January 6, 2027 |
| 1,000 days after | June 24, 2029 |
| 90 days before | June 30, 2026 |
| 12 weeks after | December 21, 2026 |
| 6 months after | March 28, 2027 |
When you move by months or years and the month you land in has no matching day, the result is set to the last day of that month. Six months after August 31, 2026 is February 28, 2027, and one year after February 29, 2024 is February 28, 2025. A note in parentheses appears below the result when this happens. For the first of those examples it reads “February has no day 31, so the last day of the month was used”. This is the same rule used for counting the months between two dates, so entering August 31, 2026 and February 28, 2027 in “Between two dates” gives 6 months 0 days.
“Amount” takes a whole number up to 100,000. If the resulting date would fall outside the years 1 to 9999, nothing is calculated and a message asks you to enter a smaller amount.
Legal deadlines may be counted differently
Laws and contracts sometimes lay down their own way of counting a period, such as whether the first day is included. This calculator applies none of those rules and simply counts calendar dates. For a date with legal consequences, such as severance pay, leave or a court deadline, confirm it with the relevant authority or a professional.