Days Calculator

Counting days by hand goes wrong the moment a month boundary is involved, and it goes badly wrong around February. This calculator counts actual calendar days, leap years included.

Two things you can do here

1. Enter a start date and an end date to count the days between them.
2. Or enter a date and a number of days to find the resulting date.
3. Use a negative number to count backwards from a deadline.
4. Read the result in days, and in weeks where that is more useful.
5. Check whether your calculation should include the start day, the end day, or neither.

The inclusive counting question

This causes more disputes than anything else in date arithmetic. From the 1st to the 10th is nine days if you are measuring elapsed time, and ten days if you are counting the days themselves — as a hotel, a rental agreement or a legal notice period might.

There is no universally correct answer; it depends on the rule you are working under. Contracts and regulations usually spell it out, and if the difference of one day matters, read that clause carefully rather than assuming.

Where day counts matter

• Notice periods in contracts, employment and tenancy agreements.
• Payment terms — net 30 means thirty days from the invoice date.
• Project deadlines and sprint planning.
• Visa validity and permitted length of stay.
• Counting down to an event, a launch or a due date.

Leap years, briefly

A year is a leap year if it divides by four, except century years, which must divide by 400. So 2024 was a leap year, 1900 was not, and 2000 was. Any calculation spanning a February needs to get this right, which is exactly why counting on your fingers is unreliable.

Date calculation questions