sundown vs time of day

sundown and time of day both are nouns.

Word	Noun	Adjective	Verb	Adverb
sundown	Yes	No	No	No
time of day	Yes	No	No	No

As nouns, time of day is a hypernym of sundown; that is, time of day is a word with a broader meaning than sundown:

sundown: the time in the evening at which the sun begins to fall below the horizon
time of day: clock time

Other hypernyms of sundown include hour.

sundown (noun)	time of day (noun)
the time in the evening at which the sun begins to fall below the horizon	clock time