How the date calculator works
Every date can be turned into a day number (days since a fixed epoch), which makes the distance between two dates a subtraction and adding days an addition. Months and years are the awkward part, because they are not a fixed number of days, so the calculator treats them as calendar steps: adding a month moves to the same day of the next month, clamped to that month's length.
days between = day number(end) - day number(start) (+1 if the end date is included) date + n months = same day of month, n months on, clamped to the month's last day
Worked example: days between
From January 1, 2026 to December 25, 2026. December 25 is day 359 of the year and January 1 is day 1, so the difference is 358 days, or 51 weeks and 1 day. Including the end date, the span is 359 days. Counting full calendar months, Jan 1 to Dec 1 is 11 months, leaving 24 days.
Business days over that span: 358 days is 51 full weeks (255 weekdays) plus one extra day. Jan 1, 2026 is a Thursday, and the extra day is the Thursday, Dec 24, so there are 256 weekdays before Dec 25, or 257 if Dec 25 (a Friday) is included. Public holidays are not removed.
Worked example: adding days
Switch the mode to add 90 days to January 1, 2026. Count month by month: 31 days reach February 1, 28 more reach March 1 (59 used), and 31 more reach April 1 (90 used). The result is Wednesday, April 1, 2026. Adding 3 months instead gives April 1 as well, but that is coincidence: adding 3 months to January 31 gives April 30, while adding 90 days to January 31 gives May 1.
Useful day counts
| Span | Days | Note |
|---|---|---|
| 1 week | 7 | |
| 30 / 60 / 90 days | 30 / 60 / 90 | Common payment and return windows; not the same as 1, 2, 3 months |
| 1 common year | 365 | 52 weeks and 1 day |
| 1 leap year | 366 | 52 weeks and 2 days; 2028 is the next |
| Average year | 365.2425 | Gregorian mean over the 400-year cycle |
| Average month | 30.4369 | 365.2425 / 12 |
Business days
Deadlines expressed in business days skip Saturdays and Sundays. Ten business days from a Monday is the Monday two weeks later, not the Thursday of the following week. The calculator counts weekdays from the start date up to (and optionally including) the end date. It does not know about public holidays, which differ by country, state, and employer, so subtract those separately.
Month-end dates
When you add or subtract months from the 29th, 30th, or 31st, the result can land in a month that is too short. The rule used here, and by most calendars, banks, and spreadsheet functions such as EDATE, is to clamp to the last day of the target month. Years behave the same way, so February 29, 2024 plus one year is February 28, 2025.
For age in years, months, and days, use the age calculator; for hours and minutes, the time calculator.