Add days to a date
To add days to a date, start with the base date and move forward by the selected number of calendar days. This calculator can also add weeks, months and years before the day adjustment is applied.
Subtract months from a date
Subtracting months uses calendar months rather than a fixed number of days. If the target month does not contain the original day number, the result is clamped to the final day of that month.
Calendar arithmetic explained
Calendar arithmetic is applied in order: years, months, weeks and days. This order matters for end-of-month dates and leap years. For example, 31 January 2026 plus 1 month becomes 28 February 2026 because February 2026 has no 31st day.