Calendar era

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A calendar era is the year numbering system used by a calendar. For example, the Gregorian calendar numbers its years in the Western Christian era (the Coptic and Ethiopic churches have their own Christian eras, see below). The instant, date, or year from which time is marked is called the epoch of the era. There are many different calendar eras. Some are listed below along with their abbreviations (if any).

A.D. (or AD) — for the Latin Anno Domini, meaning in the year of (our) Lord. This is the dominant or Western Christian Era; AD is used in the Gregorian calendar. Anno Salutis, meaning in the year of salvation is identical to the same era. Originally intended to number years from the birth of Jesus, in fact the calculation was a few years off. Traditionally, years preceding AD 1 are numbered using the BC era (see below) to avoid zero or negative numbers. However, the astronomical year numbering convention happily uses zero and negative year numbers in the AD era.
*Note: AD was also used in the medieval Julian Calendar as well, but the calendars are not identical. To distinguish between them, O.S. and N.S. were often added to the date, especially during the 17th and 18th centuries, when both calendars were in common use. Old Style (O.S.) was used for the Julian calendar. New Style (N.S.) was used for the Gregorian calendar.
*Note: AD is ambiguous about where in the calendar year the numbered year starts. While in modern times it is always January 1, the start of the calendar year, previously other dates were used, such as March 25. Many countries switched to using January 1 as the start of the numbered year when switching from the Julian calendar to the Gregorian calendar, but others switched earlier or later.
A.C., B.C. (or BC) — for the Latin Ante Christum, and the English Before Christ, respectively. Used for years prior to AD 1, counting backwards so the year n BC is the year 1-n AD. Using these two calendar eras in the intended way means that there is no year 0 or negative year numbers. However, the astronomical year numbering convention uses the AD era exclusively, so includes zero and negative year numbers.
C.E. (or CE) — meaning Common Era, which is used with the Gregorian calendar and is equivalent to the Anno Domini era. This use is similar to that of the Vulgar Era in the past.
B.C.E. (or BCE) — meaning Before the Common Era. Used with the Gregorian calendar and equivalent to B.C..
A.U.C. (or AUC) — for the Latin Ab Urbe Condita, meaning from the founding of the city (of Rome), although this was probably calculated inaccurately. The first day of its year was Founder's Day (April 21), although most modern historians assume that it coincides with the modern historical year (January 1 to December 31). It was rarely used in the Roman calendar and in the early Julian calendar — naming the two consuls that held office in a particular year was dominant. Dionysius Exiguus implied, but did not explicitly state, that AD 1 was 754 AUC, so that the year 2006 is the same as the year 2759 AUC (2006 + 753).
A.M. (or AM) — for the Latin Anno Mundi, meaning in the year of the world. This is used in the Hebrew calendar which counts years from the creation of the world, which was thought by the era's founders to have taken place in the year 3761 BC. AM was also used for all first millennium world eras of the early Christian chronographers.
Anno Lucis of Freemasonry adds 4000 years to the AD date. This was claimed to date from Creation.
Aetos Kosmou of the Byzantine Greek Calendar claims to date from Creation. The year 7514 of this era begins in September 2005 CE.
A.M. (or AM), Era of Martyrs — from the Latin Anno Martyrum (in the year of the martyrs). Used in the Coptic calendar, and equivalent to the Anno Diocletiani, counting years from the start of the reign of Diocletian, in 284.
A.H. (or AH) — for the Latinized Anno Hegirae, meaning in the year of the Hijra, Prophet Muhammad's migration from Mecca to Medina in September 622, which is taken to be the beginning of the Muslim era. This is used in the Islamic calendar. (Note that, since the Islamic calendar is a purely lunar calendar, its year count increases faster than that of solar and lunisolar calendars.)
A.H.S. (or AHS) — used by the Iranian calendar to denote the number of solar years since the Prophet Muhammad's migration from Mecca to Medina in September 622.
B.E. — for the Buddhist Era, which has an epoch (origin) of 545 BC, but with an offset of one year from this zero year the difference BE − AD is 543 in Thailand.
B.E. — for the Bahá'í Era beginning 21 March 1844. See Bahá'í calendar for more details.
B.P. — for Before Present, specifically, the number of radiocarbon years before 1950, used in radiocarbon dating.
S.E. or (SE) — for the Saka Era, used in some Hindu calendars, epoch dated from near the vernal equinox of year 78 (year 0); its usage spread to Southeast Asia before year 1000. This era is also used (together with the Gregorian calendar) in the Indian national calendar, the official civil calendar used in communiques issued by the Government of India.
Y.Z. — for the era of Yazdegerd, and is used by the Zoroastrian calendar, the religious calendar of the Zoroastrians. Its epoch is 16 June 632, the date of the coronation of Yazdegerd III of Persia, the last monarch of the Sassanid dynasty.
The Incarnation Era is used by Ethiopia. Its epoch is 29 August 8 in the Julian calendar.
The Republican Era of the French Republican Calendar was dated from 22 September 1792, the day of the proclamation of the French First Republic.
The Seleucid Era, formerly used in much of the Middle East, uses the epoch 312 BC, the year when Seleucus I Nicator captured Babylon and began his reign over the Asian portions of Alexander the Great's empire.
Chinese eras or Nian Hao were used sporadically from 156 BC and continuously from 140 BC. Until 1367 several were used during each emperor's reign. From 1368 until 1912 only one era name was used by each emperor, who was posthumously known by his era name.
*The Republican era is used by the Republic of China (Taiwan) since 1929, and assigns year one to 1912, the first year of the republic. Coincidentally, this is the same as the Juche era used in North Korea.
*Expatriate Chinese use a continuous count of years from the reign of the legendary Yellow Emperor, using the epoch 2698 BC (year 1). Western writers begin this count at either 2637 BC or 2697 BC (see Chinese calendar).
Japanese eras or Nengo were used sporadically from 645 and continuously from 701. Until 1867 several were used during each emperor's reign. From 1868 only one era name has been used by each emperor. Since 1868, each emperor has been known posthumously by his era name.
Korean eras or Dangi were used from 536 to 963 and from 1894 to 1910.
HE, the Holocene Era, is a reform of the Gregorian Calendar proposed by Cesare Emiliani with its epoch set at 10,000 BC, roughly coincident to the beginning of human civilisations. In this calendar, the year 2006 is simply 12006 HE, and the assassination of Julius Caesar in 44 BC is translated to 9957 HE.
psU, post scriptum Ulysses, is used with the Poundian calendar to number years from the completion of the novel Ulysses. The first day of the year 1 psU was November 1 1921. In general, the year n psU runs from November 1 of 1920+n CE to October 31 of 1921+n CE. (The Poundian calendar is precisely aligned with the Gregorian calendar.)

Calendar era

See also