Add or Subtract Days
The Add or Subtract Days calculator moves any date forward or backward by days, weeks, months or years. Month and year arithmetic clamps to the end of the target month, so results stay valid dates every time — January 31 plus one month becomes February 28 or 29.
What this means
When you move a date by a fixed number of days, every day counts the same. Months are different: adding one month to January 31 should land on a valid date, so it becomes the last day of February. The same clamping applies to February 29 when adding years.
How we calculate it
Formula
Result = base date ± amount in the selected unit. Months clamp to the end of the target month; a week is 7 days.Worked example
August 9, 2026 plus 90 days:
- 1August 9 + 22 days = August 31
- 2+ 30 days = September 30
- 3+ 38 more days = November 7, 2026
Important assumptions
- Month arithmetic clamps to the last day of the target month (Jan 31 + 1 month = Feb 28/29).
- A week always equals 7 days.
Frequently asked questions
What is January 31 plus one month?
February 28 (or February 29 in a leap year). The result is clamped to the last day of the target month so the date stays valid.
What happens to February 29 when adding years?
Adding one year to February 29, 2024 gives February 28, 2025, because 2025 is not a leap year. Adding four years gives February 29, 2028.
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