How to Add or Subtract Days from a Date

Adding days

To add days, count forward from your start date, and when you run past the end of the month, continue into the next one. The clean way is to use up the rest of the current month first, then carry the leftover into the following month. Add 10 days to 25 March: March has 31 days, so 6 remain in March (to the 31st), leaving 4 days that land on 4 April. Breaking it at the month boundary avoids the classic error of forgetting how many days the month actually had.

Subtracting days

Subtracting works the same way in reverse: count backward, and when you go past the first of the month, drop into the previous month, borrowing its length. To take 10 days off 5 April, use up the 4 days back to 1 April, leaving 6 more to count backward through March — which ends on the 31st — landing on 26 March. The trick, exactly as with adding, is to remember how long the month you're stepping back into actually is.

Month lengths and leap years

Every slip in date arithmetic comes back to the calendar's irregular months. They run 28, 29, 30 or 31 days, so you can't assume a fixed length as you roll from one to the next. February is the special case: 28 days most years, but 29 in a leap year — those divisible by 4, except full centuries not divisible by 400. Any span that crosses 29 February gains a day. This is precisely the fiddly bookkeeping a date calculator handles without slipping.

Calendar days vs business days

Plain date addition counts every day — Saturdays, Sundays and holidays included. That's what you want for most purposes: a birthday, an expiry date, '90 days from today'. But some deadlines are quoted in business days, meaning weekends and public holidays don't count. Those need a different calculation that skips non-working days, which is why '10 days' and '10 business days' can land on very different dates. Always check which kind a deadline means before you count.

Frequently asked questions

How do I add days to a date?

Count forward, rolling into the next month when you pass its last day.

What is 10 days after 25 March?

4 April.

Do added days include weekends?

Yes, unless you're counting business days only.