Year numbering is the assignment of integers to calendaryears for the purpose of naming the years.
A calendar defines, among many other things, the length of each year. Calendars with different year lengths must use different numbering systems.
However within a single calendar, it is possible to have several year numbering systems. This occurs for both the Gregorian calendar currently in common use and also the Julian calendar which preceded it.
Calendars with identical year lengths can share a numbering system, as has been proposed for several proposed reformed calendars based on Gregorian years.
Various systems of yearnumbering were used with the Julian calendar, starting with ab urbe condita (from the supposed founding of Rome) or the reign year of the current ruler.
Years were numbered from the supposed date of the "incarnation" or "annunciation" of Christ on March 25 of the new year 1 (offset by 753 years from ab urbe condita).
Easter, Christmas and New Year are still calculated according to the Julian calendar in the Eastern Orthodox churches, and some Eastern Orthodox churches continue to use the Julian Calendar for all their church calendar dates.
The anno Domini method of numberingyears was not widely used in Western Europe until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752.
Bede was the first historian to use a BC year and hence the first to adopt the convention of no year 0 between BC and AD, in his Historia ecclesiastica gentis Anglorum (Ecclesiastical history of the English people, 731).
Astronomical yearnumbering cannot be used to support year 2000 as the first year of the 3rd millennium because of uncertainty regarding astronomical millennia.
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