Date Tools
Add or Subtract Days
Pick a start date, enter a number of days, and choose whether to add or subtract. Get the exact result date with day of week, ISO week number, and distance from today.
How it works
The calculator adds or subtracts the number of days directly from the start date, crossing month and year boundaries correctly. Leap years are handled automatically. The result shows the ISO week number per the ISO 8601 standard, where week 1 contains the first Thursday of the year.
Examples
| Start date | Operation | Result | Note |
|---|---|---|---|
| Jan 1, 2025 | Add 90 days | Apr 1, 2025 (Tuesday) | |
| Dec 31, 2024 | Add 1 day | Jan 1, 2025 (Wednesday) | Crosses year boundary |
| Mar 1, 2024 | Subtract 1 day | Feb 29, 2024 (Thursday) | Leap year |
| Today | Add 30 days | Next month same day (approx) | Quick deadline estimate |
Frequently asked questions
Does it account for leap years?
Yes. The calculation uses JavaScript's built-in Date object, which correctly handles February 29 in leap years.
What is the maximum number of days I can add?
Up to 36,500 days (100 years) in either direction.
How is the ISO week number calculated?
Using the ISO 8601 standard: week 1 is the week containing the first Thursday of the year. Weeks start on Monday.
Is data sent to a server?
No. All calculations happen in your browser.