past vs past times

past and past times both are nouns.

Word	Noun	Adjective	Verb	Adverb
past	Yes	Yes	No	Yes
past times	Yes	No	No	No

As nouns, past and past times are synonyms defined as:

Other synonyms of past include yesteryear.

past (noun)	past times (noun)
a verb tense that expresses actions or states in the past	the time that has elapsed
the time that has elapsed
a earlier period in someone's life (especially one that they have reason to keep secret)

past (adjective)	past times (adjective)
earlier than the present time; no longer current
of a person who has held and relinquished a position or office

past (adverb)	past times (adverb)
so as to pass a given point