millennium vs time period

millennium and time period both are nouns.

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

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

Other hypernyms of millennium include period, period of time.

millennium (noun)	time period (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)