Date Plus Days Calculator
Add a number of calendar days to a start date to find the resulting date, or enter a negative number to count backward. The calculator rolls over months and years and handles leap years automatically. It also reports the day of the week and the day of the year of the result, useful for deadlines, due dates, and warranty or notice periods.
How adding days works
resulting date = start date + days (calendar days)
month and year roll over automatically
leap years (Feb 29) are counted
negative days step backward
Every calendar day counts, including weekends and holidays. The result's weekday and day-of-year are derived from the resulting date.
Worked example
Adding 90 days to 1 January 2026 gives 1 April 2026 (January has 31 days, February 28 in 2026, March 31: 31 + 28 + 31 = 90, landing on 1 April). That is the 91st day of the year.
Date plus days: frequently asked questions
How does adding days to a date work?
The calculator advances the start date by the number of calendar days you enter, rolling over month and year boundaries and accounting for leap years. Enter a negative number to step backward to an earlier date.
Are weekends and holidays included?
Yes. This adds calendar days, so weekends and holidays are counted. To add only working days, use a business days calculator instead.
Does it handle leap years?
Yes. The underlying calendar arithmetic knows that February has 29 days in leap years (years divisible by 4, except centuries not divisible by 400), so spans across 29 February are counted correctly.
What does the day of the year tell me?
The day of the year is the ordinal position of the resulting date within its year, from 1 on 1 January to 365 (or 366) on 31 December. It is handy for scheduling and record-keeping.
Sources
- U.S. Naval Observatory, astronomical applications: aa.usno.navy.mil (calendar reference).
- Day arithmetic follows the standard Gregorian calendar.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.