These are the major time periods referred to when talking about the ancient ruins, artifacts, and books. The order of the eras is not completely clear, however Nie Li does state that the Era before the Age of Darkness was the Sacred Empire Era.
At one point Nie Li mentions the Black Gold Empire Era, but then it is said that the Black Gold Empire was an empire during the Snow Wind Empire Era. 
The major eras are:
(Oldest to Youngest)
|Saint Spirit Empire|
|Snow Wind Empire|
|Woodland Empire||This is not described as a major "Era," but rather a time between the fall of the Sacred Empire and the Age of Darkness.|
|Age of Darkness (current)||The human empires have fallen and only pockets of civilization survive.|