How to use the Add or Subtract Days
- Choose the unit you want to shift by.
- Enter the starting date.
- Enter the number of units — use a negative number to go backwards.
- Read the resulting date in ISO format and in words.
- Check the weekday, since deadlines falling on a weekend usually roll forward.
How the calculation works
Adding days or weeks is arithmetic on a day count and is unambiguous. Adding months and years is not, because the target month may be shorter than the starting one. The convention used here, shared by most date libraries and by legal drafting practice, adds to the month number and lets the day overflow forwards: 31 January plus one month lands on 3 March in a non-leap year.
Some jurisdictions and contracts instead clamp to the last valid day, so 31 January plus one month becomes 28 February. Neither is wrong, but they differ by several days, so any agreement expressed in months should state which rule applies. Expressing the term in days avoids the argument entirely, which is why banking documents overwhelmingly do so.
Weekday awareness matters more than the raw date. Filing deadlines, payment terms and notice periods usually roll to the next business day when they land on a weekend or holiday, and some regimes roll backwards instead to stay within a month. The calculator reports the weekday so you can apply whichever rolling convention governs your case.
result = start ± n days, weeks (×7), months (month index shift) or years (year index shift), UTC-basedSource: Date arithmetic conventions per ISO 8601:2019 and ECMAScript Date semantics.
Worked example
A 45-day statutory response window opens on 12 November 2026.
- Start: 2026-11-12, unit days, n = 45.
- Result: 2026-12-27.
- That falls on a Sunday.
- Rolling forward to the next business day gives Monday 28 December.
The deadline computes to 27 December 2026, but under a next-business-day rule the effective date becomes Monday 28 December.
Frequently asked questions
What happens adding a month to 31 January?+
It overflows to 2 or 3 March depending on leap year. If your contract clamps to month end instead, subtract the overflow manually.
How do I subtract?+
Enter a negative number. Minus 30 days moves the date back a month.
Does it skip weekends?+
No, this one counts every calendar day. Use the business days calculator for working-day arithmetic.
Why work in UTC?+
So that daylight saving transitions cannot add or remove an hour and accidentally shift the date.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.