past vs by

past is a noun but by is not a noun.

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

As adverbs, past and by are synonyms defined as:

past (noun)	by (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)

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earlier than the present time; no longer current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

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