The calculator answers two questions about calendar dates. “Between dates” counts the time from a start date to an end date. “Add / subtract” moves a date forward or back by a number of days, weeks, months, years or business days.
Type dates as YYYY-MM-DD or pick them with the calendar button. The Today button fills in the current date from your browser. Between two dates you can also enter times as HH:mm. The result shows the span in years, months and days, the total days, weeks and days, business days (Monday to Friday) and an ISO 8601 duration. Every date shown also gets its weekday, ISO week and day of the year. The inputs are kept in the page address, so you can share a result by copying the link.
Example
Between dates, start 2026-03-01, end 2026-03-31, “Include end date” off:
- Result: 30 days
- Years, months, days: 0 years, 0 months, 30 days
- Weeks: 4 weeks 2 days
- Business days (Mon–Fri): 21
- ISO 8601 duration: P30D
With “Include end date” on, the same dates give 31 days and 22 business days.
Add / subtract, date 2026-01-31, add 1 month:
- Result: 2026-02-28 (Saturday)
- Note: February 2026 has no day 31, so the result is the last day of the month, the 28th.
- Details: week 2026-W09, day 59 of the year
An end date before the start date gives a negative result, marked with a minus sign.
Limitations
- Public holidays are not known, and the weekend is always Saturday and Sunday.
- Dates are calendar dates without a time zone, so daylight saving time never changes a result.
- Years run from 0001 to 9999. A result outside that range is rejected.
- The proleptic Gregorian calendar is used for all years. Dates before the Gregorian reform of 1582 are not converted from the Julian calendar, so a historical date may differ from the one in old sources.
- “Include end date” is ignored when you enter a time, because the times already mark exact moments.
FAQ
Is the end date included?
By default it is not. From 1 March to 31 March is 30 days, and business days stop before the end date. Tick “Include end date” to count both the first and the last day, which gives 31.
What happens when I add a month to January 31?
February has no day 31, so the result is the last day of February: 2026-01-31 plus 1 month is 2026-02-28, and 2024-01-31 plus 1 month is 2024-02-29. The result tells you when this happened. The same rule applies to years: 2024-02-29 plus 1 year is 2025-02-28.
Are public holidays counted as business days?
Yes, they are counted as ordinary weekdays, because holidays differ by country and year. Only Saturdays and Sundays are skipped. Adding business days from a weekend counts from the next Monday as day 1; subtracting counts back from the previous Friday.
Why does a year have week 53?
ISO 8601 numbers weeks from Monday, and week 1 is the week that contains the year’s first Thursday. A year therefore has 53 weeks when 1 January or 31 December is a Thursday. 2026-12-31 is 2026-W53, 2021-01-01 still belongs to 2020-W53, and 2024-12-30 is already 2025-W01.
Which years are leap years?
A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 and 2024 are leap years and 1900 and 2026 are not.