millennial vs millennian

millennial and millennian both are adjectives.

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

As adjectives, millennial and millennian are synonyms defined as:

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

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