The Earth takes a few minutes less than 365-1/4 days (365.24219) to go completely around the Sun. Once every four years we add a day to the Gregorian calendar — February 29 — to make up for most of this difference. That’s a leap year.

However, because the difference is a few minutes short of 1/4 day, any year that is evenly divided by 100 would not be a leap year unless it is evenly divided by 400. Thus 1600, 2000, and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200, and 2300 are common years, even though they are all divisible by 4.