past vs time immemorial

past and time immemorial both are nouns.

Word	Noun	Adjective	Verb	Adverb
past	Yes	Yes	No	Yes
time immemorial	Yes	No	No	No

As nouns, time immemorial is a hyponym of past; that is, time immemorial is a word with a more specific, narrower meaning than past:

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

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earlier than the present time; no longer current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

past (adverb)	time immemorial (adverb)
so as to pass a given point