bimillennium vs period of time

bimillennium and period of time both are nouns.

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

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

Other hypernyms of bimillennium include period, time period.

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