How to count the days between two dates

Counting days sounds trivial until the answer matters — a notice period, a visa, a pregnancy week, a deadline. Most disagreements come from four questions: whether both ends count, how long each month is, whether a leap day falls in between, and whether weekends count.

1. Do you count the start and end dates?

The standard difference counts the nights between two dates: from Monday to Wednesday is 2 days. If both days themselves count — a hotel stay counted in days, or “from the 1st to the 3rd inclusive” — add one: 3 days.

Contracts and laws usually specify which they mean. When they don’t, the exclusive count (end minus start) is the common default, and it is what you get when you subtract dates in a spreadsheet.

2. Month lengths and a worked example

Count the days left in the first month, add whole months in between, then add the days in the last month.

From 15 January 2026 to 1 March 2026:

1. January: 31 − 15 = 16 days remaining after the 15th
2. February 2026: 28 days (2026 is not a leap year)
3. March: 1 day to reach the 1st
4. Total: 16 + 28 + 1 = 45 days (46 if both ends are counted)

3. Leap years

A year is a leap year, with 29 February, if it is divisible by 4 — except century years, which must be divisible by 400.

• 2024 and 2028 are leap years.
• 1900 and 2100 are not.
• 2000 was.

From 28 February to 1 March is 2 days in 2024 and 1 day in 2025. Any count that crosses February needs to know the year.

4. Weekdays and business days

Business-day counts usually exclude Saturdays and Sundays. From Thursday 15 January 2026 to Sunday 1 March 2026 there are 45 days in total, of which 32 are weekdays. Public holidays differ by country and region, so subtract the ones that apply to you.

5. “One month later” is ambiguous

Adding days is exact; adding months is not, because months differ in length. The usual convention — used by our calculators and most software — keeps the day number and, if that day doesn’t exist, uses the last day of the month: one month after 31 January is 28 February (29 in a leap year). Adding another month gives 28 March, not 31 March.

Ages follow the same rule. Someone born on 29 February turns a year older on 28 February in common years under this convention; some countries’ laws use 1 March instead.

6. Time zones and clocks

Counting calendar dates should not depend on the clock. Software that subtracts timestamps can be off by one around daylight-saving changes, when a “day” lasts 23 or 25 hours. Our date calculators work with calendar dates only, so the result is the same wherever you are.