An Epoch is any remarkable date from which time is reckoned. It signifies a stop. ASC 17.1
An Era is any indefinite series of years commencing at a known epoch. It signifies time-the time from a stop. ASC 17.2
A Period is a definite series of years. The date of independence is the Epoch from which the era of the independence is reckoned. The President is elected for a Period of four years. ASC 17.3
Epochs, eras, and periods, are to chronology what the cardinal points and prime meridians are to astronomy and geography. As on celestial or terrestrial maps or globes, the relative position of stars or places cannot be given without some known point to reckon from, so in chronology given points are needed by which to arrange the relative distances of events. ASC 17.4