The Gregorian calendar is the calendar used by most of the world. Also called the “Christian calendar” or “Western calendar,” it is internationally accepted as a civil calendar by all but a handful of countries. The Gregorian calendar was introduced in 1582 primarily to fix errors in the Julian calendar mostly having to do with leap years.
In the Julian calendar, named after Julius Caesar, every fourth year had 366 days rather than 365. Roman astronomers calculated that a year — the time it takes the Earth to revolve around the sun — had a duration of 365.25 days. This method of adding a “leap day” every fourth year averaged out to this determined value.