millennium vs period of time

millennium and period of time both are nouns.

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

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

Other hypernyms of millennium include period, time period.

millennium (noun)	period of time (noun)
a span of 1000 years	an amount of time
(New Testament) in the Book of Revelation it is foretold that those faithful to Jesus will reign with Jesus over the earth for a thousand years; the meaning of these words have been much debated; some denominations (e.g. Jehovah's Witnesses) expect it to be a thousand years of justice and peace and happiness
the 1000th anniversary (or the celebration of it)