Add or Subtract Days Calculators
Find the date a set number of days, weeks or months away.
Add or Subtract Days is a free online tool that find the date a set number of days, weeks or months away. It runs entirely in your web browser using plain JavaScript, so files are processed on your own device and never uploaded to a server. There is no sign-up, no file size limit imposed by the site, no watermark and no paid tier.
- Price
- Free — no account, no quota, no watermark
- Category
- Calculators
- Where it runs
- In your browser, on your device
- Files uploaded
- None
- Technology
- plain JavaScript
- Settings
- 5
- Works offline
- Yes, after the first visit
About Add or Subtract Days
Adding months is not the same as adding 30 days, and this uses calendar arithmetic: 31 January plus one month lands on 2 or 3 March, because February has no 31st and the overflow rolls forward. That behavior matches how most date libraries work, and it does trip people up.
How to use it
- Enter start date, amount and unit.
- Adjust the remaining 2 settings if the defaults do not fit your case.
The 5 settings
| Setting | What it does | Default |
|---|---|---|
| Start date | Free text. | — |
| Amount | A numeric value. | 30 |
| Unit | Choose from 4: Days, Weeks, Months, Years. | Days |
| Direction | Choose from 2: Add, Subtract. | Add |
| Count working days only | On or off. Off by default. | Off |
Under the hood
| Runs on | plain JavaScript — runs the whole thing |
| Controls | Start date, Amount, Unit, Direction, Count working days only |
Questions
How does the working-days option count?
It steps forward one day at a time and only decrements the counter on Monday to Friday, so weekends are skipped rather than subtracted at the end. Public holidays are not included.
What does the unit option change?
It offers 4 choices — Days, Weeks, Months and Years. The default is Days, which suits most cases; change it when your input or your destination calls for something specific.
Is the arithmetic exact?
It uses double-precision floating point — about 15 significant figures, far more than the inputs justify. Displayed rounding is cosmetic; the underlying numbers are not rounded between steps, so chained results do not drift.