The leap year rule in three steps
A year is a leap year if it divides evenly by 4 — with one exception. Century years (those ending in 00) are only leap years when they also divide by 400. That correction keeps the calendar in step with the 365.2425-day solar year.
- Divisible by 4? Usually a leap year.
- Divisible by 100? Then not a leap year …
- … unless divisible by 400. Then it is a leap year after all.
So 2024 is a leap year, 1900 is not, and 2000 is.
Days in each month
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 or 29 | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
Only four months have 30 days — April, June, September and November. February is the only one shorter than 30. The rest have 31. To check on your hand, count across your knuckles: a knuckle is a 31-day month, the dip between is 30 (or 28 for February).
The day-of-year number
The ordinal date counts the day's position in the year: January 1 is day 1 and December 31 is day 365, or 366 in a leap year. It is used in logistics, IT and astronomy, and it makes "days remaining in the year" a simple subtraction.
Common questions
How many days are in a year?
365 in a common year and 366 in a leap year. The average across the calendar is 365.2425 days, which is why one extra day is added roughly every four years.
Why does February have 28 days?
February was the last month of the early Roman calendar and was left short so the year came out to the right length. It keeps 28 days in a common year and gains a 29th on February 29 in leap years.
Is 2100 a leap year?
No. 2100 divides by 100 but not by 400, so the leap year is skipped. 2000 was a leap year because it divides by 400; 1900 was not.
