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Gregorian calendar, solar dating system now in general use. It was proclaimed in 1582 by Pope Gregory XIII as a reform of the Julian calendar.

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Within a year, the change had been adopted by the Italian states, Portugal, Spain, and the Roman Catholic German states. Gradually, other countries adopted the Gregorian calendar: the Protestant German states in 1699, Great Britain and its colonies in 1752, Sweden in 1753, Japan in 1873, China in 1912, the Soviet socialist republics in 1918, and Greece in 1923. Islamic countries tend to use the Gregorian calendar for secular life but retain calendars based on Islam for religious purposes (see Islamic calendar).

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The Gregorian calendar is a solar dating system used by most of the world. It is named for Pope Gregory XIII, who issued the papal bull Inter gravissimas in 1582, announcing calendar reforms for all of Catholic Christendom. The Gregorian calendar modified the existing Julian calendar, which Julius Caesar had introduced to the ancient Roman Republic in 46 BCE.

By the Julian reckoning, the solar year comprised 365 1/4 days, and the intercalation of a “leap day” every four years was intended to maintain correspondence between the calendar and the seasons. A slight inaccuracy in the measurement (the solar year comprising more precisely 365 days, 5 hours, 48 minutes, 45.25 seconds) caused the calendar dates of the seasons to regress almost one day per century.

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Although the Julian calendar was the dominant European dating system for more than 1,600 years, its solar year measurements (365.25 days versus the more precise 365.2422 days) contained a slight inaccuracy that caused the calendar’s seasonal dates to regress nearly one day per century. Pope Gregory XIII introduced calendar reforms in 1582 to correct the issue. The Gregorian calendar continues the preexisting system of leap years to realign the calendar with the Sun, but no century year is a leap year unless it is exactly divisible by 400.

Although this regression had amounted to 14 days by Pope Gregory’s time, he based his reform on restoration of the vernal equinox, then falling on March 11, to March 21, the date it occurred in 325 ce, which was the time of the First Council of Nicaea, and not the date of the equinox at the time of the birth of Christ, when it fell on March 25. The change was effected by advancing the calendar 10 days after October 4, 1582, the day following being reckoned as October 15.

The Gregorian calendar was adopted by much of Catholic Europe in 1582, as directed by Pope Gregory XIII in the papal bull Inter gravissimas, which was published in February of that year. Protestant and Eastern Orthodox countries initially refused to abide by the new calendar, and the reformed system was foreign to countries outside the European sphere of influence. Over time, however, the Gregorian calendar was adopted for civil purposes by most countries around the world. Learn more.

The Gregorian calendar differs from the Julian only in that no century year is a leap year unless it is exactly divisible by 400 (e.g., 1600 and 2000). A further proposed refinement, the designation of years evenly divisible by 4,000 as common (not leap) years, would keep the Gregorian calendar accurate to within one day in 20,000 years.