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The Julian calendar , proposed by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar.[1] It took effect on 1 January 45 BC (AUC 709), by edict. It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII. The Julian calendar gains against the mean tropical year at the rate of one day in 128 years. For the Gregorian calendar, the figure is one day in 3,030 years.[2] The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%, being 10.8 minutes longer.

The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long. Consequently the Julian year drifts over time with respect to the tropical (solar) year. Although Greek astronomers had known, at least since Hipparchus, a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was corrected by the Gregorian reform of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years.[3] This means since 16 February Julian (1 March 1900 Gregorian) and until 15 February Julian (28 February 2100 GregorianJulian is 13 days behind Gregorian and the disparity will widen.

The Julian calendar has been replaced as the civil calendar by the Gregorian calendar in all countries which officially used it.[4] An analog, the Alexandrian calendar, is the basis for the Ethiopian calendar, which is the civil calendar of Ethiopia. Egypt converted on 20 December 1874/1 January 1875. Turkey switched (for fiscal purposes) on 16 February/1 March 1917. Russia changed on 1/14 February 1918.[5] Greece made the change for civil purposes on 16 February/1 March 1923, but the next national day (25 March)—a religious holiday—took place as if on the old calendar. Most Christian denominations in the west and areas evangelised by western churches have made the same change for their liturgical calendars.

Most branches of the Eastern Orthodox Church use the Julian calendar for calculating the date of Easter, upon which the timing of all the other moveable feasts depends. Some such churches have adopted the Revised Julian calendar for the observance of fixed feasts, while such Orthodox churches retain the Julian calendar for all purposes.[6] The Julian calendar is still used by the Berbers of the Maghreb in the form of the Berber calendar,[7] and on Mount Athos.

During the changeover between calendars and for some time afterwards, dual dating was used in documents and gave the date according to both systems. In contemporary as well as modern texts that describe events during the period of change, it is customary to clarify to which calendar a given date refers by using an O.S. or N.S. suffix (denoting Old Style, Julian or New Style, Gregorian).

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