Add Days to a Date Calculator
Free "add days to date" calculator — pick a start date and a number of days (positive or negative) to find the resulting date. Handles leap years.
Pick a start date and the number of days to add (negative to subtract). The calculator returns the resulting date.
What “add N days” really means
Adding days is calendar arithmetic, not “add N × 86,400 seconds and hope DST doesn’t move things”. We do the math at midnight UTC so daylight- saving transitions don’t shift the answer by a day at the boundary.
Includes / excludes
The start date is day zero. So “30 days after Jan 1” is Jan 31, not Jan 30. Want inclusive counting (start counts as day 1)? Subtract 1 from the days field.
Leap years
Real Gregorian calendar math:
- 2024, 2028, 2032 are leap years (Feb 29 exists).
- 2025, 2026, 2027 are not.
- Century years follow the 400-year rule: 2000 was a leap year, 2100 will not be.
The calculator handles all this transparently.
Business days vs calendar days
This calculator counts every day, weekends and holidays included. A proper business-days helper has to know the country’s holiday list, which varies; we may add it as a separate page.
Worked examples
-
30 days after 2026-01-01
30 days after 2026-01-01 → 2026-01-31
-
1 day before 2026-01-01
1 day before 2026-01-01 → 2025-12-31
-
365 days after 2026-01-01
365 days after 2026-01-01 → 2027-01-01
Frequently asked questions
How does the calculator handle leap years?
It uses real calendar math, not 365 × N. February 29 exists in leap years (2024, 2028, 2032, …); adding 1 day to 2024-02-28 returns 2024-02-29. Adding 1 year to a Feb 29 birthday this way crosses to March 1 in non-leap years.
Does it include or exclude the start date?
Excludes. "30 days after 2026-01-01" is January 31, not January 30. The start date is day zero of the count. If you need inclusive counting, subtract 1 from the days field.
What about weekends and holidays?
Pure calendar days, including weekends and any holiday. We may add a separate "business days" calculator later, since holiday lists are country-specific.
How far can I go in either direction?
Effectively any reasonable range — millions of days in either direction stay accurate. Calendar reform dates (pre-1582 Gregorian switchover) are handled by the JavaScript Date API as proleptic Gregorian, so dates before the actual switchover are notional.
Why UTC under the hood?
To prevent daylight-saving transitions in your local time zone from shifting the answer by a day at the boundary. The result is a pure calendar-day count, independent of zones.
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