What Is a Leap Year?

Nearly every four years is a leap year which has 366 days, as opposed to a common year, which has 365 in the Gregorian Calendar.

Illustration image — Leap years have 366 days, not 365.
We use leap years to sync our common years with the tropical years.

When Is the Next Leap Year?

Next leap day is February 29, 2020.

Last leap day was February 29, 2016.

Why Add Leap Years?

Leap years are needed to keep our modern day Gregorian calendar in alignment with the Earth's revolutions around the sun.

It takes the Earth approximately 365.242189 days – or 365 days, 5 hours, 48 minutes, and 45 seconds – to circle once around the Sun. This is called a tropical year, and is measured from the March equinox.

However, the Gregorian calendar has only 365 days in a year, so if we didn't add a leap day on February 29 nearly every four years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by around 24 days!

Is There a Perfect Calendar?

Exactly Which Years Are Leap Years?

We add a Leap Day on February 29, almost every four years. The leap day is an extra, or intercalary, day and we add it to the shortest month of the year, February.

In the Gregorian calendar three criteria must be taken into account to identify leap years:

The year can be evenly divided by 4;
If the year can be evenly divided by 100, it is NOT a leap year, unless;
The year is also evenly divisible by 400. Then it is a leap year.

This means that in the Gregorian calendar, the years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years.

Special Leap Year 2000

The year 2000 was somewhat special as it was the first instance when the third criterion was used in most parts of the world since the transition from the Julian to the Gregorian Calendar.

Who Invented Leap Years?

Roman general Julius Caesar introduced the first leap years over 2000 years ago. But the Julian calendar had only one rule: any year evenly divisible by four would be a leap year.

This formula produced way too many leap years, but was not corrected until the introduction of the Gregorian calendar more than 1500 years later.