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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.

In antiquity, regnal years were counted from the ascension of a monarch. This makes the Chronology of the ancient Near East very difficult to reconstruct, based on disparate and scattered king lists, such as the Sumerian King List or the Babylonian Canon of Kings. In East Asia, reckoning by era names appointed by ruling monarchs remained current down to the 20th century (in Japan, down to the present day).

The first "universal" calendar era not dependent on the ruling monarch was introduced in the 4th century BC, with the olympiad, counted from the first Olympic Games in 776 BC.

Hellenistic periodEdit

  • Olympiads, fixed at 776 BC, in use from the 4th c. BC down to the 3rd or 4th century AD; the "modern olympiad" is set at 1896.
  • 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. The era was in use from the 3rd century BC down to the 6th century AD.
  • A.U.C. (or AUC) — for the Latin ab urbe condita, meaning from the founding of the city (of Rome), was introduced by Marcus Terentius Varro in the 1st century BC, 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 2017 is the same as the year 2770 AUC (2017 + 753).

Late Antiquity and Middle AgesEdit

Most of the traditional calendar eras in use today were introduced at the time of transition from Late Antiquity to the Early Middle Ages, roughly between the 6th and 10th centuries.


Dionysian "Common Era"Edit

Main article: Anno Domini

The era based on the date of the birth of Christ was introduced by Dionysius Exiguus in the 6th century and is in continued use with various reforms and derivations.

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.

  • 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.
  • B.C. (or BC) — Before Christ. 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, Equivalent to the Anno Domini era. This use is similar to that of Era Vulgaris (or EV) in the past. B.C.E. (or BCE) — meaning Before the Common Era. Equivalent to B.C..

Some modern calendar eras are direclty tied to the Dionysian era:


  • 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.



The Zoroastrian calendar used regnal years since the reform by Ardashir I, but after the fall of the Sassanid empire, the ascension of the last Sassanid ruler, Yazdegerd III of Persia, coronated June 16 632, continued to be used as reference year, abbreviated Y.Z. or "Yazdegerd era".


  • A.M. (or AM) — for the Latin Anno Mundi, meaning in the year of the world, set to the year 3761 BC. This was introduced to the Hebrew calendar in ca. the 9th century[1], based on calculations of the date of creation by Jose ben Halafta (2nd century).






  1. Encyclopedia Britannica (2007), s.v. Jewish Era

See alsoEdit

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