Add or Subtract Days from a Date
The date a given number of days before or after a start date.
Results
Resulting date
17 Dec 2026
Day of the week
4
ISO week number
51
What this tool does
Deadlines, notice periods and guarantee expiries are all counted in plain calendar days. Enter a negative number to go backwards, and note that the result includes every day, not just working days.
Formula
resultant date = initial date + n days
Variables
| Symbol | Meaning | Unit |
|---|---|---|
start_date | Start date | — |
add_days | Days to add (negative to subtract) | — |
date | Resulting date | — |
weekday | Day of the week | — |
week | ISO week number | — |
Worked example
- Start date2026-09-18
- Days to add (negative to subtract)90
- Resulting date17 Dec 2026
- Day of the week4
- ISO week number51
Limitations
- Dates are handled as calendar dates. Time zones, daylight saving changes and public holidays are not taken into account.