epoch vs period of time

epoch and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
epoch	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of epoch; that is, period of time is a word with a broader meaning than epoch:

epoch: a period marked by distinctive character or reckoned from a fixed point or event
period of time: an amount of time

Other hypernyms of epoch include period, time period.

epoch (noun)	period of time (noun)
a period marked by distinctive character or reckoned from a fixed point or event	an amount of time
a unit of geological time that is a subdivision of a period and is itself divided into ages
(astronomy) an arbitrarily fixed date that is the point in time relative to which information (as coordinates of a celestial body) is recorded