Gregorian calendar replaces Julian calendar
The Gregorian calendar is the calendar used in most parts of the world. It was introduced on February 24 with a papal bull, and went into effect in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.
The rule for leap years is: