Date Add / Subtract Calculator
Add or subtract days, weeks, months, or years from any date.
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VR
Reviewed by
Vikram RaoSenior Software Engineer
· B.Tech Computer Science· 14 years building timezone-aware systems· Open-source contributor to date-fns
About the Date Add / Subtract Calculator
Adding or subtracting time from a date sounds simple, but months have different lengths (28–31 days) and years have different lengths (365 vs 366 days in leap years). This means "add one month" cannot always mean "add 30 days" — doing so would cause end-of-month dates to drift incorrectly.
This calculator uses proper calendar arithmetic: when you add months, it moves to the same day-of-month in the target month, clamping to the last valid day when necessary. For example, January 31 + 1 month = February 28 (not March 2). When you add years, it preserves the month and day — with February 29 clamping to February 28 in non-leap target years.
Month-End Edge Cases Explained
| Operation | Result | Why |
|---|---|---|
| Jan 31 + 1 month | Feb 28 | Feb has 28 days; result clamps to last day |
| Jan 31 + 1 month (leap year) | Feb 29 | Feb has 29 days in a leap year |
| Mar 31 − 1 month | Feb 28 | Feb has no 31st; clamps to 28th |
| Feb 29 + 1 year | Feb 28 | Next year may not be a leap year |
| Jan 31 + 30 days | Mar 2 | Day arithmetic is exact; no clamping applies |
Common Use Cases
- ✓Payment due dates — Calculate when a 30-day invoice falls due, or when a 12-month subscription automatically renews.
- ✓Contract & notice periods — Determine the exact end date of a 90-day notice period, a 2-year lease, or a 3-month probationary period.
- ✓Expiry & shelf life — Find when a product, prescription, or professional certification expires if issued on a known date.
- ✓Scheduling recurring events — Plan quarterly reviews, annual renewals, or monthly check-ins by projecting dates weeks, months, or years forward.
- ✓Construction & project planning — Add build-phase durations to a project start date to compute milestone and completion dates.
Frequently Asked Questions
Enter your base date, select ''Add'' as the operation, enter the number of days, and select ''Days'' as the unit. The calculator instantly shows the resulting date with full details.
Yes. Select ''Subtract'' as the operation and choose your unit (days, weeks, months, or years). The calculator computes the exact resulting date accounting for month lengths and leap years.
Select ''Months'' as the unit and enter the number of months to add. The calculator handles variable month lengths correctly — for example, adding 1 month to January 31 gives February 28 (or 29 in a leap year).
ISO 8601 date format is YYYY-MM-DD (e.g. 2026-03-15). It is an international standard for representing dates unambiguously. The calculator shows your result in ISO format for easy use in code and databases.
The number of days in 3 months varies (roughly 90–92 days) because months have different lengths. Use the date add calculator: enter today''s date, select Add, enter 3, and choose Months to get the exact resulting date.
Adding 1 month to January 31 gives February 28 in a common year, or February 29 in a leap year. The calculator clamps to the last valid day of the target month rather than overflowing into March, which would be incorrect.
Enter today''s date as the base date, select ''Add'', enter 90, and choose ''Days''. The result shows the exact date 90 days from now, along with the day of the week and ISO date string.
Yes. Select ''Subtract'' as the operation, enter 6, and choose ''Months''. The calculator will compute the date exactly 6 months before your base date, correctly handling month-end edge cases.
Adding 1 year always lands on the same calendar date one year later (e.g., March 15, 2025 → March 15, 2026). Adding 365 days can give a different result if a leap day falls in between. Use ''Years'' for anniversary-style calculations.