Most of us know that February gets an extra day every four years, but the math behind leap years is actually slightly more complicated to keep our calendar in perfect sync with the seasons.
The Solar Year is Not 365 Days
A standard calendar year has 365 days. However, it takes the Earth approximately 365.2422 days to orbit the sun. If we just ignored that extra quarter of a day, our calendar would slowly drift out of alignment with the seasons. After a century, summer would start weeks earlier!
The 3 Rules of Leap Years
To account for the .2422 drift, we add an extra day to February. But adding one day every four years (0.25) actually overcorrects slightly. Therefore, the Gregorian calendar uses three rules to determine if a year is a leap year:
- Rule 1: A year may be a leap year if it is evenly divisible by 4.
- Rule 2: However, years divisible by 100 are NOT leap years (e.g., 1700, 1800, 1900).
- Rule 3: The exception to Rule 2 is that years divisible by 400 ARE leap years (e.g., 1600, 2000).
Calculating Your Exact Age?
Because of leap years, your age in exact days isn't just (Age × 365). Use our calculator to find out exactly how many days old you are, factoring in every leap day you have lived through.
Open Exact Age CalculatorWhat If We Stopped Using Leap Years?
Without leap years, the calendar dates of the equinoxes and solstices would shift backwards by about one day every four years. In less than 400 years, New Year's Day in the Northern Hemisphere would happen in autumn instead of winter!