Why February 28 plus one day is not always March 1
Date addition follows the days that actually exist in the calendar. Leap days and year boundaries need calendar rules rather than manual guesses.
Add days to date →Compare a leap year with an ordinary year
Adding one day to 2024-02-28 gives 2024-02-29. Adding another day reaches March 1. In contrast, adding one day to 2025-02-28 gives 2025-03-01 immediately. Matching month-and-day inputs can therefore lead to different month-and-day outputs when the years differ. That is expected calendar behavior, not inconsistent arithmetic.
Year boundaries follow the same principle
Adding one day to 2026-12-31 gives 2027-01-01, while subtracting one gives 2026-12-30. Simply editing the day number can leave an impossible value such as December 32. The calculator resolves the carry or borrowing into a valid date automatically, which is particularly helpful for longer offsets.
Know the supported calendar scope
The site accepts valid Gregorian dates and limits results to years 1 through 9999. It does not calculate festivals in other calendar systems or interpret one calendar day as an exact time-zone-aware 24-hour duration. For completed age, use the age calculator instead of substituting a fixed day count for an anniversary. The date-difference tool can check an addition by measuring the resulting interval.
Try the calculators
- Add days to date
Find the date before or after a number of days.
- Date difference
Count calendar days between two dates.
- Age calculator
Find completed years and days between dates.