Date Add / Subtract
Add or subtract days, weeks, or months from a date.
Use on any page (bookmarklet)
Want Date Add / Subtract without leaving the page you're on? Drag the button below to your bookmarks bar, then click it on any website to open Date Add / Subtract right there — it runs entirely in your browser.
Use responsibly: a bookmarklet runs on whatever page you click it on. Avoid sensitive sites such as online banking, payment, or healthcare pages — you run it at your own risk. Everything is processed locally and no data is sent anywhere. See our Terms.
- Show your bookmarks bar if it's hidden — Ctrl+Shift+B (⌘+Shift+B on Mac).
- Drag the button above onto the bookmarks bar.
- Open any website and click the bookmark — the Date Add / Subtract panel appears in the top-right corner. Use ✛ to move it between corners, or ›/‹ to tuck it against the edge and pull it back out.
- Click the bookmark again (or the ✕) to close it.
Note: a few sites with strict security policies may block bookmarklets.
Examples
Add 30 days
Input
2025-01-01 + 30 days
Output
2025-01-31 (Friday)
Subtract 3 months
Input
2025-06-15 − 3 months
Output
2025-03-15 (Saturday)
About this tool
This free online date calculator adds or subtracts days, weeks, months, or years from any date. Pick a start date, choose add or subtract, enter an amount and unit, and the new date appears instantly with its day of the week. Everything runs in your browser with no upload.
How to use
- Choose a start date.
- Pick Add or Subtract and enter an amount.
- Choose the unit — days, weeks, months, or years.
Month arithmetic is where dates get strange
Adding days is unambiguous. Adding months is not, because months are different lengths and some target dates simply do not exist. Add one month to 31 January and there is no 31 February to land on. This tool follows standard JavaScript behaviour and rolls the overflow forward:
- 31 Jan 2025 + 1 month → 3 March (February had 28 days)
- 31 Jan 2024 + 1 month → 2 March (leap year, 29 days)
- 31 May + 1 month → 1 July (June has only 30 days)
- 29 Feb 2024 + 1 year → 1 March 2025
Other systems clamp instead, returning 28 February for the first case. Neither convention is wrong, but they disagree — so whenever a start date falls near the end of a month, check the result rather than assuming.
Adding and subtracting are not mirrors
A consequence of that rollover: reversing an operation does not always return you to where you started. 31 January plus one month gives 3 March, and 3 March minus one month gives 3 February — not 31 January. For the same reason, applying two offsets in a different order can give a different date, since each month step depends on where in the calendar you currently sit. When a rule specifies an order, follow it literally.
“30 days” is not “one month”
These are different units and produce different dates from almost every start point. A payment term of “net 30” means 30 calendar days; a subscription renewing “monthly” lands on the same date each month regardless of length. Use whichever unit the contract actually names — the difference is up to three days per step, which compounds over a year.
Note too that every unit here is calendar-based, so a computed deadline can land on a weekend or a public holiday. The weekday shown alongside the result is the quickest way to catch that.
Common uses
Finding a deadline a set number of days out, working out renewal and expiry dates, scheduling follow-ups and reminders, counting back from a due date to a start date, and checking notice periods. To measure the gap between two dates you already have, use date difference, and for recurring weekly planning the week number tool maps dates onto ISO weeks.
Frequently asked questions
How do I add days to a date?
Pick a start date, choose Add, enter the number of days, and the result date appears instantly — along with the day of the week it lands on.
Can I subtract weeks, months, or years?
Yes. Choose Subtract and pick the unit — days, weeks, months, or years — to count backwards from your start date. Subtracting is not always a perfect mirror of adding: add one month to 31 January and you land in early March, but subtracting one month from that result does not return you to 31 January.
Why doesn't 31 January plus one month give 28 February?
Because there is no 31 February to land on, and something has to give. This tool follows standard JavaScript date behaviour, which rolls the overflow forward into the next month — so you get 2 or 3 March depending on the year. Other systems clamp to the last valid day instead and return 28 February. Neither is more correct; they are different conventions, which is precisely why month arithmetic is worth double-checking whenever the start date is near the end of a month.
Is "30 days" the same as "one month"?
Only in February, and only outside a leap year. A month is a variable unit between 28 and 31 days, so a contract saying "30 days" and one saying "one month" produce different dates for most start points. When a deadline matters, use the unit the document actually specifies rather than the one that's easier to count.
Does the order of operations matter?
Yes, and this catches people out. Adding one month then 30 days does not always equal adding 30 days then one month, because the month step depends on where in the calendar you currently are. If you need to apply two offsets, do them in the order the rule specifies rather than combining them.
Does it count business days?
No — every unit here is calendar-based, so a result can land on a weekend or public holiday. If your deadline is defined in working days, add the calendar equivalent (roughly the working days divided by 5 and multiplied by 7) and then check the weekday shown in the result.
Does it tell me the weekday of the result?
Yes. The result shows the new date together with its day of the week, which is the fastest way to spot that a computed deadline has landed on a Saturday and needs moving.
How are leap years handled when adding years?
Adding one year to 29 February has the same overflow problem as month-end dates, since the target year usually has no 29 February. Add a year to 29 February 2024 and you land on 1 March 2025 rather than 28 February. For anniversaries and renewals falling on that date, most contracts specify which convention applies.
Is my data uploaded anywhere?
No. The calculation runs entirely in your browser with no upload.
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