How to Count the Days Between Two Dates Correctly

Counting days sounds trivial until you have to decide whether to include the first day, cross a February or span a leap year. These rules keep your count right.

Inclusive or exclusive?

The most common mistake is being off by one. From Monday to Friday of the same week:

• Exclusive (the difference): 4 days. This is what most date tools return, and what you want for "how many days until...".
• Inclusive (counting both ends): 5 days. Use this when the start and end days both count, such as a 5-day course or a leave request.

Hotel nights are always the exclusive count: check in on the 3rd and out on the 7th, and you've stayed 4 nights.

Month lengths

Months have 28 to 31 days. The knuckle trick helps: make a fist and count across your knuckles and the dips between them. Every knuckle is a 31-day month (January, March, May, July, August, October, December) and every dip is a shorter month.

Leap years

February has 29 days in a leap year. The rule:

• A year divisible by 4 is a leap year,
• except years divisible by 100, which are not,
• except years divisible by 400, which are.

So 2024 and 2028 are leap years and 2000 was one, but 1900 and 2100 are not.

Counting across months

Break the span into pieces. From March 20 to May 5:

• March 20 to March 31: 11 days
• March 31 to April 30: 30 days
• April 30 to May 5: 5 days

Total: 46 days (exclusive).

Week-based shortcuts

Any multiple of 7 days lands on the same weekday. To find the weekday 100 days from today, divide by 7: 100 days is 14 weeks and 2 days, so it falls two weekdays after today.

Business days

For working days, count the full weeks (5 working days each), add the leftover weekdays, then subtract any public holidays in that span. Holidays vary by country and region, so check your local calendar.