Add or Subtract Days Calculator

Pick a start date, choose whether to add or subtract, then enter the number of years, months, weeks, and days. The calculator applies the offset using the calendar (not just a flat day count) and returns the resulting date, its day of the week, and the ISO format. For example, June 25, 2026 plus 30 days lands on July 25, 2026.

Years and months are applied first as a single month shift, then weeks and days are added as one day shift. This order matches Python's dateutil relativedelta and SQL DATEADD. It matters because the month step clamps to the end of a short month before any days are added, so the day count is never thrown off by an intermediate rollover.

The result is clamped to the last day of the target month. For example, January 31 plus one month becomes February 28, or February 29 in a leap year, because neither month has a 31st. This end-of-month rule is the standard convention used by spreadsheet and database date functions.

Yes. It uses the proleptic Gregorian calendar, where a year is a leap year if it is divisible by 4, except for century years that are not divisible by 400. February 29 is treated correctly when you cross a leap year, and a February 29 start date clamps to February 28 in a non-leap target year.

Weeks and days are added by counting forward or backward on the calendar, so they roll over month and year boundaries automatically. For example, December 20, 2025 plus 2 weeks lands on January 3, 2026. The calculator shows the net number of days between your start date and the result so you can confirm the span.

Yes. Set the direction to subtract and enter the offset as positive numbers. The calculator moves backward from the start date and reports the result with a negative net day count. This is useful for finding a deadline a set number of days before an event or a date a fixed period in the past.