Epoch is the displacement from zero at zero time of a body undergoing simple harmonic motion(S.H.M).
its unit is milliseconds.

hope i helped u:)
plzzzzzzzz mark as best if helpful
An epoch is the beginning of a distinctive period in the history of something or someone, whereas an era is a period of history characterized by particular circumstances or by a particular series of events. A period is a more general term for a distinct portion of time in relation to a person's life or to human history and an aeon is an immeasurable length of time often used in more rhetorical contexts. In geology, the three terms epochera, and period have special meanings: era denotes the largest unit of time, a period is a division of this, and an epoch is a subdivision of a period.