Gregorian calendar

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The Gregorian calendar is the calendar currently used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years slightly differently to make the average calendar year 365+97⁄400, or 365.2425, days long rather than the Julian calendar's 365+1⁄4 days, thus 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 that every year divisible by four is a leap year, except for years that are divisible by 100 but not also divisible by 400. For example, 1800 and 1900 were not leap years, but 1600 and 2000 were. There were two reasons to establish the Gregorian calendar. First, the Julian calendar was based on the estimate that the average solar year is exactly 365.25 days long, an overestimate...

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