A calendar era is the year numbering system used by a calendar. The Gregorian calendar for example numbers its years in the Western Anno Domini system (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras). The instant, date, or year from which time is marked is called the epoch of the era. There are many different calendar eras such as Saka Era.
In ancient times, regnal years were counted from the year a new monarch took the throne. This makes ancient timelines very difficult to reconstruct, because they are based on separate and scattered king lists, such as the Sumerian King List and the Babylonian Canon of Kings. In East Asia, having era names based on ruling monarchs ended in the 20th century except for Japan, where they are still used.