How to use the Date Add/Subtract Calculator
- Choose the start date and whether to add or subtract.
- Fill in any of years, months, weeks and days.
- Press Calculate.
Formula
Years and months are applied first as calendar months. If the day doesn’t exist in the target month (e.g. 31 January + 1 month), the last day of that month is used. Weeks and days are then added as exact days.
Examples
- 31 January 2027 + 1 month = 28 February 2027.
- 26 October 2026 + 1 year 6 months = 26 April 2028.
Frequently asked questions
Is adding 12 months the same as adding 365 days?
Not quite: over a leap day, 12 months is 366 days.