era vs period of time

era and period of time both are nouns.

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

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

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

Other hypernyms of era include period, time period.

era (noun)	period of time (noun)
a period marked by distinctive character or reckoned from a fixed point or event	an amount of time
a major division of geological time; an era is usually divided into two or more periods