twenty-four hour period vs noonday

twenty-four hour period and noonday both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-four hour period	Yes	No	No	No
noonday	Yes	No	No	No

twenty-four hour period (noun)	noonday (noun)
time for Earth to make a complete rotation on its axis	the middle of the day