Periods, eras (6)
I believe the answer is:
epochs
I believe this clue is a double definition.
'periods' is the first definition.
(I have seen 'Historical periods ' mean 'epochs' so perhaps 'periods' could also mean 'epochs')
'eras' is the second definition.
(I've seen this before)
(Other definitions for epochs that I've seen before include "dates" , "Periods of historical time" , "Particular periods of time" , "Divisions of geological periods" , "Remarkable period of time" .)