[5][6] The Latin word use in chronology seems to have begun in 5th century Visigothic Spain, where it appears in the History of Isidore of Seville,[7] and in later texts.
[10] Like epoch, "era" in English originally meant "the starting point of an age"; the meaning "system of chronological notation" is c. 1646; that of "historical period" is 1741.
A "calendar era" indicates a span of many years which are numbered beginning at a specific reference date (epoch),[12] which often marks the origin of a political state or cosmology, dynasty, ruler, the birth of a leader, or another significant historical or mythological event;[13] it is generally called after its focus accordingly as in "Victorian era".
[15] There are currently three eras defined in the Phanerozoic; the following table lists them from youngest to oldest (BP is an abbreviation for "before present").
[23] In the Western church, Anno Domini (AD also written CE), counting the years since the birth of Jesus on traditional calculations, was always dominant.
[32] The use of regnal years is a rather impractical system, and a challenge for historians if a single piece of the historical chronology is missing, and often reflects the preponderance in public life of an absolute ruler in many ancient cultures.
Such traditions sometimes outlive the political power of the throne, and may even be based on mythological events or rulers who may not have existed (for example Rome numbering from the rule of Romulus and Remus).