How to count days between dates: inclusive, exclusive and working days
Asking how many days there are between two dates sounds simple, yet two people can honestly give different answers. The difference is almost always whether the first and last day are counted.
Exclusive and inclusive counting
The usual calculation subtracts the earlier date from the later one. From 1 January to 31 December 2026 that is 364 days: the start date is day zero and the end date is day 364. Counting both ends gives 365, which is what you want when you ask how many days an event covers. A course that runs Monday to Wednesday covers 3 days, but the difference between the two dates is 2.
Use the days between dates calculator and tick Count the end date too to switch between them.
Leap years
A year has 366 days if it is divisible by 4, except century years, which must be divisible by 400. So 2024 and 2000 were leap years and 1900 was not. From 1 January 2024 to 1 January 2025 is 366 days.
Years, months and days
Age and contract periods are quoted in years, months and days. The method is to add whole calendar months to the start date for as long as the result does not pass the end date, then count the days left. Someone born on 15 May 1990 is, on 4 November 2026, 36 years, 5 months and 20 days old: 15 May 1990 plus 36 years and 5 months is 15 October 2026, and 20 days later is 4 November.
When the day does not exist in a month, such as the 31st in a 30-day month, the date moves to the last day of that month. Try it with the age calculator or the date calculator.
Working days
Working days exclude Saturdays and Sundays. From Monday 2 November to Monday 9 November 2026 there are 7 days but 5 working days. Public holidays differ by country and region, so they are not subtracted; take them off by hand.
29 February birthdays
A leap-day birthday has no exact anniversary in three years out of four. Calculators here treat 28 February as the anniversary. Some countries' laws use 1 March instead, so use the rule that applies where it matters legally.