This article is about a calendar used for civil and liturgical purposes. For the day-number calendar used for astronomical and historical calculations, see
. For day of year, see
||January 24, 2017
||January 11, 2017
The Julian calendar, proposed by
Julius Caesar in 46 BC (708
AUC), was a reform of the
Roman calendar. It took effect on 1 January 45 BC (AUC 709) (shortly before the
Roman conquest of Egypt), 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 the figure is one day in 3,030 years.
 The difference in the average length of the
year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.
The Julian calendar has a regular year of 365 days divided into 12 months, as listed in the
table below. A
leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long. It was intended to approximate 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, years evenly divisible by 100 are not leap years, except that years evenly divisible by 400 remain leap years.
 Consequently, the Julian calendar is currently (since 14 March 1900 Gregorian/1 March Julian and until 28 February 2100 Gregorian/15 February Julian) 13 days behind the Gregorian calendar.
The Julian calendar
has been replaced as the
civil calendar by the Gregorian calendar in almost all countries which formerly used it, although it continued to be the civil calendar of some countries into the 20th century.
 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.
 Greece made the change for civil purposes on 16 February/1 March 1923 - the national day (25 March), which was a religious holiday, was to remain on the old calendar. Most
Christian denominations in the west and areas evangelised by western churches have also replaced the Julian calendar with the Gregorian as the basis for their
liturgical calendars. However, most branches of the Eastern
Orthodox Church still use the Julian calendar for
calculating the date of Easter, upon which the timing of all the other
moveable feasts depends. Some Orthodox churches have adopted the
Revised Julian calendar for the observance of
fixed feasts, while other Orthodox churches retain the Julian calendar for all purposes.
 The Julian calendar is still used by the
Berbers of the Maghreb in the form of the
 and on
Mount Athos. In the form of the
Alexandrian calendar, it is the basis for the
Ethiopian calendar, which is the civil calendar of Ethiopia.
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).