Counting down to a holiday, working out a notice period, or checking how long ago something happened all depend on the same skill: finding the number of days between two dates. It sounds simple, but calendars have uneven months and leap years, so it is easy to be off by a day. Here is how to do it properly.
Method 1: Count by months
This works well for dates in the same year. Write down how many days are left in the first month, add the full days of any months in between, then add the day number of the last month.
Example: from 10 March to 25 May. Days left in March after the 10th: 31 − 10 = 21. All of April: 30. Days in May: 25. Total: 21 + 30 + 25 = 76 days.
Method 2: Use day-of-year numbers
Every date has a number from 1 to 365 (366 in leap years). Subtract the earlier number from the later one. If the dates are in different years, add the length of the full years in between.
Should you include the end date?
This is the most common source of mistakes. If you count the days that pass between two dates, Monday to Wednesday is 2 days. If you count every day on the calendar including both the start and end, the same period is 3 days. Hotels count nights, which is the first kind. Some leave forms and contracts count both days, which is the second kind. Check which rule your situation uses.
Counting working days
Business days are normally Monday to Friday. A rough shortcut is to multiply the number of full weeks by 5, then count the remaining days one at a time, skipping Saturdays and Sundays. Public holidays differ by country and employer, so subtract them separately.
Leap years
A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2024 and 2000 were leap years, but 1900 and 2100 are not. February has 29 days in a leap year.
Handy facts
- A common year has 365 days, which is 52 weeks and 1 day.
- 30 days have September, April, June and November. February has 28 or 29, and all the others have 31.
- 100 days from today is about 14 weeks and 2 days away.
To avoid arithmetic mistakes, use our free Days Between Dates Calculator. It gives total days, weeks, months and business days, and lets you include the end date. You can also find your exact age with the Age Calculator.
Frequently asked questions
How many weeks is 90 days?
12 weeks and 6 days.
How do I add days to a date?
Add the days to the day number and roll over into the next month whenever you pass the end of the month.
This guide is for general information only. See our disclaimer.