bimillennium vs time period

bimillennium and time period both are nouns.

Word	Noun	Adjective	Verb	Adverb
bimillennium	Yes	No	No	No
time period	Yes	No	No	No

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

Other hypernyms of bimillennium include period, period of time.

bimillennium (noun)	time period (noun)
a span of 2000 years	an amount of time
the 2000th anniversary (or the celebration of it)