Add or Subtract Days Calculator — Free Online
Add or subtract any number of days, weeks, months, or years from any date. Instantly find the resulting date with weekday name — perfect for deadlines and planning.
How to Use the Add or Subtract Days Calculator
- Select your starting date using the date picker (defaults to today)
- Choose "Add" to move forward in time or "Subtract" to move backward
- Enter the number of units to add or subtract
- Select the unit: Days, Weeks, Months, or Years
- Click "Calculate Date" — the result shows the exact date, weekday, and ISO format
How the Add/Subtract Days Calculator Works — The Formula
The calculator converts your start date to an internal day count, adds or subtracts the specified duration, then converts back to a calendar date. Months and years are handled using calendar arithmetic (not fixed day counts) to correctly handle varying month lengths.
For days/weeks: Result = StartDate + (Amount × DaysPerUnit)
For months: Result = calendar month offset
(January 31 + 1 month = February 28/29)
For years: Result = same day/month in target year
(Feb 29 in leap year → Feb 28 in non-leap year)
Formula validated by Vipul Jaganiya, Computer Science expert in Calendar Mathematics and Timezone Engineering.
Real-World Examples
| Start Date | Operation | Amount | Result |
|---|---|---|---|
| 2026-05-26 | Add | 30 days | 2026-06-25 (Thursday) |
| 2026-05-26 | Add | 90 days | 2026-08-24 (Monday) |
| 2026-12-25 | Subtract | 1 month | 2026-11-25 (Wednesday) |
| 2026-01-01 | Add | 1 year | 2027-01-01 (Friday) |