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Concept
Epoch
Résumé
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.
The Yoruba calendar (Kọ́jọ́dá) uses 8042 BC as the epoch, regarded as the year of the creation of Ile-Ife by the god Obatala, also regarded as the creation of the earth.
Anno Mundi (years since the creation of the world) is used in the Byzantine calendar (5509 BC).
Anno Mundi (years since the creation of the world) as used in the Hebrew calendar (3761 BC).
The Mesoamerican Long Count Calendar uses the creation of the fourth world in 3114 BC.
Olympiads, the ancient Greek era of four-year periods between Olympic Games, beginning in 776 BC.
Ab urbe condita (753 BC), used to some extent by Roman calendars of the Roman imperial period.
Buddhist calendars tend to use the epoch of 544 BC (date of Buddha's parinirvana).
The term Hindu calendar may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is the Vikram Samvat (58 BC), also used in modern times as the national calendars of Nepal and Bangladesh.
The Julian and Gregorian calendars use as epoch the Incarnation of Jesus as calculated in the 6th century by Dionysius Exiguus. (Subsequent research has shown that this was not the best estimate for the date of birth of Jesus.) This epoch was applied retrospectively to the Julian calendar, long after its original creation by Julius Caesar.
The Islamic calendar counts "lunar years" by Anno Hegiræ (in the year of the Hijra) or AH era (AD 622). The year count shifts relative to the solar year as the calendar is purely lunar.
Source officielle
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