The history of Tamriel, and the lands beyond, is divided into six discreet eras.
The Dawn era is a time of gods, where events are often nonlinear and contradictory. The Merethic has linear time, but very few remaining records and a lot of conflicting information. Time during the Merethic is also counted in reverse, starting with ME2500 and ending with ME0. The First era is when proper history begins.
Elder Scrolls Online and Elder Scrolls Adventures: Redguard take place in the 2nd era, Elder Scrolls V: Skyrim takes place in the 4th era, and the rest of the games take place during the 3rd.
You can see each detailed era timeline in the links below.
