Add or Subtract Days Calculator
The date a number of days before or after a start date.
Input sheet
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Whether you are counting forward to a project deadline or backward to find when paperwork had to be filed, adding or subtracting days from a fixed start date removes the guesswork that comes from mentally jumping across month boundaries and leap years.
How it works
Adds (or subtracts, with a negative value) the given number of days to the start date.
The calculator works in whole calendar days, so it automatically handles the uneven length of months and the extra day in a leap year. February 28 plus three days lands on March 3 in a normal year and March 2 in a leap year without you having to remember which is which.
Because it counts calendar days rather than business days, every Saturday and Sunday is included. Enter a negative number to walk backward in time from the start date instead of forward.
Resulting date = start date + number of days (a negative number subtracts days). Weeks shown = absolute value of days ÷ 7.
Worked examples
Start date January 1, 2026, add 90 days. → April 1, 2026 (a Wednesday), 12.9 weeks later
January has 31 days, February 28, and March 31, totaling 90 days, which lands exactly on April 1.
Start date March 15, 2026, subtract 30 days. → February 13, 2026
Entering -30 walks back across the 15 days of early March and the rest into February.
Tips & gotchas
- Use a negative value to find a past date, such as the cutoff for a 30-day return window measured back from today.
- Calendar-day math counts weekends and holidays; if you need only working days, use the Business Days calculator instead.
- For monthly milestones like a 6-month anniversary, adding 180 days is an approximation; true month arithmetic can differ by a day or two depending on month lengths.
- Double-check the day of the week shown if the deadline must avoid landing on a weekend.
FAQ
Does this account for leap years?
Yes. The calculation uses real calendar dates, so February 29 is included automatically in leap years and the resulting date stays accurate across year boundaries.
How do I find a date in the past?
Enter a negative number of days. For example, -45 returns the date 45 days before your start date.
Are weekends and holidays counted?
Every calendar day is counted, including weekends and public holidays. To count only Monday through Friday, use the Business Days calculator.
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Business Days Calculator · Time Duration Calculator · Time Zone Offset Calculator · Week Number Calculator · Countdown to Date Calculator