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 chosen by ruling monarchs ceased in the 20th century, except for Japan, where they are still used.

Among the ancient Greeks, a common method for indicating the passage of years was based on the order of Olympic Games, first held in 776 BC. The pan-Hellenic games provided the various independent city-states a mutually recognizable system of dates. The first Olympiad also marks the traditional beginning of Greek historical civilization and record-keeping, and it continues to be regarded as the end of Western prehistory and the beginning of its historical epoch.