The Gregorian calendar is the calendar used in most of the world. It is named after Pope Gregory XIII, who introduced it in October 1582.
The calendar spaces leap years to make its average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.
The calendar was a revision of the Julian calendar and had two aspects. It shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes. To deal with the drift since the Julian calendar was fixed, the date was advanced 10 days; Thursday 4 October 1582 was followed by Friday 15 October 1582. There was continuity in the cycle of weekdays and the Anno Domini calendar era. The reform also altered the lunar cycle used by the Church to calculate the date for Easter (computus), restoring it to the time of the year as originally celebrated by the early Church. This calendar era has the alternative secular name of "Common Era".
The reform was adopted initially by the Catholic countries of Europe and their overseas possessions. Over the next three centuries, the Protestant and Eastern Orthodox countries also moved to what they called the Improved calendar, with Greece being the last European country to adopt the calendar in 1923. To unambiguously specify a date during the transition period (or in history texts), dual dating is sometimes used to specify Old Style and New Style dates (abbreviated as O.S and N.S.). During the 20th century, most non-Western countries also adopted the calendar, at least for civil purposes.
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