millennian vs millennial

millennian and millennial both are adjectives.

Word	Noun	Adjective	Verb	Adverb
millennian	No	Yes	No	No
millennial	No	Yes	No	No

As adjectives, millennian and millennial are synonyms defined as:

millennian and millennial: relating to a millennium or span of a thousand years

millennian (adjective)	millennial (adjective)
relating to a millennium or span of a thousand years	relating to a millennium or span of a thousand years