How to use Add / Subtract Days from Date
The date add/subtract calculator lets you add or subtract any combination of years, months, weeks, days, hours and minutes to or from a starting date and time. It handles month-end edge cases correctly (e.g. adding one month to January 31 yields the last day of February) and shows the result date, Unix timestamp and days from today. Quick presets for common intervals like +30 days, +1 year or −6 months are provided.
- Enter or select the starting date and time.
- Choose + Add or − Subtract mode.
- Enter the number of years, months, weeks, days, hours and/or minutes to apply.
- The result date updates instantly. Use quick presets for common intervals.
- Swap the result into the start field for chained calculations.
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How month arithmetic handles edge cases
Adding months is not always as simple as adding 30 days. January 31 + 1 month should yield "end of February" (28th or 29th), not March 2nd or 3rd. This calculator clamps to the last valid day of the result month, matching behaviour used by most programming languages and spreadsheet applications.
Worked examples
Month-end clamp
Inputs: Jan 31, 2025 + 1 month
Result: Feb 28, 2025 (last valid day of February)
Add 90 days
Inputs: Mar 15, 2025 + 90 days
Result: Jun 13, 2025
Subtract 6 months
Inputs: Aug 10, 2025 − 6 months
Result: Feb 10, 2025
Glossary
- Unix timestamp
- The number of seconds elapsed since 1 January 1970 00:00:00 UTC.
- ISO 8601
- The international date format standard: YYYY-MM-DD.
- Calendar arithmetic
- Date math that respects real month lengths and leap years instead of fixed 30-day assumptions.
- Leap year
- A year with 366 days, where February has 29 days, affecting date-add and date-difference results.
Related reading
Frequently Asked Questions
Why use Add / Subtract Days from Date?
- Add or subtract years, months, weeks and days in a single combined operation
- Correctly clamps month-end overflows so January 31 + 1 month gives the last day of February
- Shows the result as a Unix timestamp alongside the human-readable date
- Quick presets for +7, +30, +90 days and +1 year eliminate repetitive entry
Common use cases
- Find the date 90 days from a contract signing to set a review deadline
- Calculate when a subscription starting today will renew after exactly 6 months
- Determine a visa expiry date by adding the permitted stay in days to the entry date
- Work out when a 30-day free trial ends, including month-end edge cases
- Chain calculations by swapping the result back into the start field to find a second deadline
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