A Leap Day is an extra day on February 29 which is added nearly every 4 years to today’s Gregorian calendar. A “leapling” is a person born in a leap year.

Here are the rules for leap year, just to set the record straight.

A year is a leap year if it is divisible by 4, but century years are not leap years unless they are divisible by 400.

So, the years 1700, 1800, and 1900 were not leap years, but the year 2000 was. Non-leap years begin and end on the same day of the week.

Why Do We Need Leap Years?

One orbit of Earth around the Sun takes 365.2422 days—a little more than our Gregorian calendar’s 365. Adding an extra day, aka a leap day, to the calendar every 4 years brings the calendar in line and therefore synchronizes with the four seasons.

Without leap days, the calendar would be off by 5 hours, 48 minutes, 45 seconds each year. After 100 years, the seasons would be off by 25 days. The extra leap day adjusts this drift.

But it’s not a perfect match: Adding a leap day every 4 years overcompensates by a few extra seconds each leap year, adding up to about 3 extra days every 10,000 years.