Gregorian calendar

For the calendar of religious holidays and periods, see Liturgical year. For this year's Gregorian calendar, see Common year starting on Sunday.
2017 in various calendars
Gregorian calendar 2017
Ab urbe condita 2770
Armenian calendar 1466
Assyrian calendar 6767
Bahá'í calendar 173–174
Bengali calendar 1424
Berber calendar 2967
British Regnal year 65  Eliz. 2 – 66  Eliz. 2
Buddhist calendar 2561
Burmese calendar 1379
Byzantine calendar 7525–7526
Chinese calendar 丙申(Fire  Monkey)
4713 or 4653
    — to —
丁酉年 (Fire  Rooster)
4714 or 4654
Coptic calendar 1733–1734
Discordian calendar 3183
Ethiopian calendar 2009–2010
Hebrew calendar 5777–5778
Hindu calendars
 - Vikram Samvat 2073–2074
 - Shaka Samvat 1938–1939
 - Kali Yuga 5117–5118
Holocene calendar 12017
Igbo calendar 1017–1018
Iranian calendar 1395–1396
Islamic calendar 1438–1439
Japanese calendar Heisei 29
Javanese calendar 1950–1951
Juche calendar 106
Julian calendar Gregorian minus 13 days
Korean calendar 4350
Minguo calendar ROC 106
Nanakshahi calendar 549
Thai solar calendar 2560
Unix time 1483228800 – 1514764799

The Gregorian calendar is internationally the most widely used civil calendar. [1] [2] [3] It is named after Pope Gregory XIII, who introduced it in October 1582.

The calendar was a refinement to the Julian calendar [4] involving a 0.002% correction in the length of the year. The motivation for the reform was to stop the drift of the calendar with respect to the equinoxes and solstices—particularly the vernal equinox, which set the date for Easter celebrations. Transition to the Gregorian calendar would restore the holiday to the time of the year in which it was celebrated when introduced by the early Church. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923.

The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory XIII's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter. The reform was a modification of a proposal made by Aloysius Lilius. [5] His proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.

The Gregorian reform modified the Julian calendar's scheme of leap years as follows:

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. [6]

In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths. The canonical Easter tables were devised at the end of the third century, when the vernal equinox fell either on 20 March or 21 March depending on the year's position in the leap year cycle. As the rule was that the full moon preceding Easter was not to precede the equinox, the date was fixed at 21 March for computational purposes and the earliest date for Easter was fixed at 22 March. The Gregorian calendar reproduced these conditions by removing ten days. [7]

To unambiguously specify the date, dual dating or Old Style (O.S.) and New Style (N.S.) are sometimes used with dates. Dual dating uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates. Old Style and New Style (N.S.) indicate either whether the start of the Julian year has been adjusted to start on 1 January (N.S.) even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.) rather than the Gregorian (N.S.).

The Gregorian calendar continued to use the previous calendar era (year-numbering system), which counts years from the traditional date of the nativity ( Anno Domini), originally calculated in the 6th century by Dionysius Exiguus. [8] This year-numbering system, also known as Dionysian era or Common Era, is the predominant international standard today. [9]


A year is divided into twelve months
No. Name Length in days
1 January 31
2 February 28 or 29
3 March 31
4 April 30
5 May 31
6 June 30
7 July 31
8 August 31
9 September 30
10 October 31
11 November 30
12 December 31

The Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days, but as in the Julian calendar, in a leap year, a leap day is added to February. In the Julian calendar a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years. In the Julian calendar, this leap day was inserted by doubling 24 February, and the Gregorian reform did not change the date of the leap day. In the modern period, it has become customary to number the days from the beginning of the month, and 29 February is often considered as the leap day. Some churches, notably the Roman Catholic Church, delay February festivals after the 23rd by one day in leap years. [10]

Gregorian years are identified by consecutive year numbers. [11] The cycles repeat completely every 146,097 days, which equals 400 years. [12] [13] Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is 365 days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. [14]

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1). Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar (see the "beginning of the year" section below).