Date Calculator: Add or Subtract Time
Add or subtract years, months, weeks, days, hours and minutes to a date. Shows the resulting weekday and ISO date and explains month-end clamping.
What is Date Calculator?
Finds the date that is a given amount of time before or after a start date. You can combine years, months, weeks, days, hours and minutes, for example 1 year, 2 months and 10 days.
How to use Date Calculator
- Enter the Start date and, if needed, a Start time.
- Choose Add (+) or Subtract (−) as the operation.
- Fill in the years, months, weeks, days, hours and minutes to apply.
- Read the resulting date with its weekday and the ISO 8601 value; a note appears if the day had to be clamped to the end of a month.
Example
2026-01-31 + 1 month
Saturday, February 28, 2026 (2026-02-28) – day 31 clamped to the end of February
When to use Date Calculator
Use it for due dates, warranty or trial expiry, payment terms or ‘what date is 100 days from now’. For working days only (skipping weekends and holidays), use the business days calculator.
How Date Calculator works
Years and months are applied first: the month number is shifted and, if the original day does not exist in the target month, it is clamped to the last day (31 January + 1 month = 28 February, or 29 in a leap year). Weeks and days are then added as exact day counts, and finally hours and minutes, which can roll over into the next or previous day. Calculation uses UTC day numbers, so time zones and DST never move the result.
Common Use Cases
- Finding the date 90 days after an invoice
- Calculating a due date or expiry date
- Working out when a 6-month probation period ends
- Going back a number of weeks from an event
FAQs
What happens when I add a month to 31 January?
February has no 31st, so the result is clamped to the last day of February. The tool shows a note when this happens.
In which order are the values applied?
Years and months first, then weeks and days, then hours and minutes. Changing the order could give a different result around month ends.
Is the start date counted as day 0?
Yes. Adding 1 day to 10 March gives 11 March.