"A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, it ...
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is the year 2021 as per the Gregorian calendar, which numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).
In antiquity, regnal years were counted from the accession 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 and the Babylonian Canon of Kings. In East Asia, reckoning by era names chosen by ruling monarchs ceased in the 20th century except for Japan, where they are still used." - (en.wikipedia.org 18.09.2021)
