March 24, 2024 • 3 min Read time
Everything you need to know about leap years. Learn why our calendar needs an extra day every four years and how it is calculated.
Felix Schmidt
Mathematician & Author
What is a Leap Year?
A leap year has 366 days instead of the usual 365. The extra day is added to the end of February – February 29th. This is necessary to synchronize our calendar with the solar year (the time it takes for the Earth to orbit the Sun).
Why do we need Leap Years?
The Earth takes approximately 365.2422 days to complete one orbit around the Sun. If we strictly used 365 days every year, our calendar would shift by nearly a full day every four years. After 100 years, that would be 24 days! Summer in July would eventually occur in the middle of winter.
The Leap Year Rule
The calculation is not as simple as "every four years." The exact global rule is:
- A year is a leap year if it is divisible by 4.
- Exception: If the year is divisible by 100, it is not a leap year.
- Exception to the exception: If the year is divisible by 400, it is a leap year after all.
That's why the year 2000 was a leap year, but the year 1900 was not.
Date Calculator