past times vs yesteryear

past times and yesteryear both are nouns.

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

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

Other synonyms of past times include past.

past times (noun)	yesteryear (noun)
the time that has elapsed	the time that has elapsed