Add/Subtract Days Calculator

Adding or subtracting days from a calendar date shifts the date forward or backward along the timeline, automatically crossing month and year boundaries. The operation is straightforward arithmetic on the day count, with the calendar handling variable month lengths: January has 31 days, February has 28 or 29, April has 30, and so on. No manual month-end adjustments are needed.

Example: start on March 15, 2026 and add 45 days. March has 16 days remaining after the 15th (31 − 15 = 16). Subtract 16 from 45: 29 days left, landing on March 31. April has 30 days, so 29 days from April 1 is April 29, 2026. Subtracting works in reverse: April 29 minus 45 days returns to March 15.

Leap years affect February crossings. Adding 365 days to March 1, 2023 reaches March 1, 2024 (2024 is a leap year, but the same calendar date one year later). Adding 366 days to March 1, 2023 reaches March 2, 2024 because 2024's extra day shifts the anniversary. Adding 365 days to February 28, 2023 (non-leap) lands on February 28, 2024; from February 28, 2024 (leap year) plus 365 days is February 28, 2025.

Healthcare uses this constantly: Naegele's rule adds 280 days to the last menstrual period for a due date. Payment terms add 30 or 90 days to invoice dates. Subscription renewals add billing-cycle days. Project management adds duration days to kickoff dates. Negative values subtract — "90 days before December 31" finds the start of a quarter.

Enter a start date and the number of days to add (positive) or subtract (negative). The calculator returns the resulting date and the day of the week, handling all month-end and leap-year transitions automatically.