P.M. vs post meridiem

P.M. is not an adjective while post meridiem is an adjective.

Word	Noun	Adjective	Verb	Adverb
P.M.	No	No	No	Yes
post meridiem	No	Yes	No	Yes

As adverbs, P.M. and post meridiem are synonyms defined as:

P.M. (adjective)	post meridiem (adjective)
	after noon

P.M. (adverb)	post meridiem (adverb)
between noon and midnight	between noon and midnight