How the date calculator works
- Pick a start date (the Today button fills in today’s date).
- Choose Add or Subtract.
- Enter any combination of years, months, weeks and days.
The result updates as you type. Next to the date you get its weekday, the ISO 8601 form (for example 2026-06-13), the ISO week number, the day of the year (such as day 164 of 365) and the distance in days from the start date.
How months and years are added
Adding days and weeks is simple counting on the calendar. Months and years are different, because months have different lengths. This calendar calculator keeps the same day of the month when it exists, and otherwise uses the last day of the target month:
- 31 January 2026 + 1 month = 28 February 2026
- 31 January 2024 + 1 month = 29 February 2024 (leap year)
- 31 August 2026 + 6 months = 28 February 2027
- 31 March 2026 − 1 month = 28 February 2026
- 29 February 2024 + 1 year = 28 February 2025
When this happens, a notice tells you that the day was adjusted because, for example, 31 February does not exist.
Years and months are applied together first, then weeks and days. This is a common convention in calendar software, and it keeps the original day of the month for as long as possible.
Examples
| Start date | Operation | Result |
|---|---|---|
| 15 March 2026 | + 90 days | Saturday 13 June 2026 (ISO week 24, day 164) |
| 10 October 2026 | − 6 weeks | Saturday 29 August 2026 |
| 15 April 2026 | + 1 year, 6 months, 2 weeks | Friday 29 October 2027 |
| 31 January 2026 | + 1 month, 1 day | Sunday 1 March 2026 |
For the third example, 1 year and 6 months take 15 April 2026 to 15 October 2027, and 2 weeks (14 days) later is 29 October 2027.
Common uses
- Finding a deadline such as 30, 60 or 90 days after an invoice or a notice.
- Working out a due date or an expiry date a number of months ahead.
- Going back a number of weeks to find when something started.
- Using it as a date adder for project plans, warranties or subscriptions.
Good to know
- Days here are calendar days, weekends included. To skip weekends and public holidays, count business days instead.
- To measure the gap between two dates you already know, count the days between two dates.
- All dates use the Gregorian calendar with no time zone effects, so the arithmetic is exact. The calculator accepts dates from year 1 to 9999.