So here’s how you can most accurately describe our leap year system:
A leap year occurs in every year that is divisible by 4, except for years that are divisible by 100 and not by 400. To clarify, 1600 and 2000 were leap years, whereas 1700, 1800, and 1900 were not. This is because the Gregorian calendar repeats itself every 400 years.
Sure, it’s pretty frustrating, but leap years ensure that Earth routinely realigns its position in the Solar System, thus maintaining the seasons in their appropriate times of year.
Not all countries and cultures use the Gregorian calendar, however, so different systems account for leaps in different ways.
Some notable examples are the Chinese calendar, the Hebrew calendar, and the Baha’i Calendar. Countries that have not formally adopted the Gregorian calendar are Saudi Arabia, Ethiopia, Afghanistan, and Iran. Other nations use modified versions of the Gregorian calendar, as well as a unique calendar alongside the Gregorian one.
These other systems have their own leap days, years, or months. For example, in the Hebrew calendar, an extra (13th) month is added 7 times every 19 years.