The Avatar series does not give a unified date system. Very limited hints in the show seem to indicate a system of era names using the Avatar Cycle, which we have accepted here. The BG (Before [the Air Nomad] Genocide) and AG (After [the Air Nomad] Genocide) era system is not used in-universe but dates in this wiki should be given using this system for the sake of simplicity and compatibility with the Avatar fandom wiki. In universe, characters should not reference BG/AG, and should either use the Avatar Cycle era system described below, or a culture-specific year designation (e.g. years of the reign of an earth monarch, the incumbent fire lord, etc.)
There is a year zero AG.
A tool for keeping track of the various dating systems and lives of various important figures can be found here.
Avatar Cycle Era System
Four Elements Epoch
During this epoch, four elements prevailed. There have been Avatars of Balance prior to these, though they are of uncertain historicity.
See Avatar for a list of historical Avatars.
Reigns of the Earth Monarchs
See Dynastic History of the Earth Kingdom.
Reigns of the Fire Lords