In short: Adding days to a date counts forward (or backward) a number of calendar days, weeks, months or business days to find the resulting date and weekday.
About Add or Subtract Days from a Date
“Payment due in 30 days”, “returns accepted within 14 days”, “the offer is valid for 10 business days” — deadlines are usually stated as a count from a date, and working them out on a calendar is tedious.
This calculator adds or subtracts days, weeks, months, years or business days from any start date. Month arithmetic keeps the day of the month where possible and moves to the last day when needed (31 January + 1 month = 28 or 29 February). Business days skip Saturdays and Sundays; public holidays are not taken into account because they vary by country.
How to use Add or Subtract Days from a Date
- Pick the start date (today by default).
- Choose add or subtract and enter the amount.
- Choose the unit — days, business days, weeks, months or years.
- Read the resulting date and its weekday.
Examples
An invoice dated 5 November 2026 with 30-day terms is due on Saturday, 5 December 2026.
Ten business days after Monday 2 March 2026 is Monday 16 March 2026.
Frequently asked questions
Does “30 days” include the start date?
No. Adding 30 days to the 1st gives the 31st, which is the usual convention for deadlines.
What happens when adding a month to the 31st?
If the target month is shorter, the result is the last day of that month.
Are public holidays skipped?
No, only weekends. Add extra days for holidays in your country.
Can I count backwards?
Yes. Choose Subtract to find the date a given time before the start date.
Updated November 2026 · Free, no signup · How we build and test tools