by vs past

by is not a noun while past is a noun.

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

As adverbs, by and past are synonyms defined as:

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

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	earlier than the present time; no longer current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

by (adverb)	past (adverb)
so as to pass a given point	so as to pass a given point
in reserve; not for immediate use