sunset vs time of day

sunset and time of day both are nouns.

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

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

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

Other hypernyms of sunset include hour.

sunset (noun)	time of day (noun)
the daily event of the sun sinking below the horizon	clock time
atmospheric phenomena accompanying the daily disappearance of the sun
the time in the evening at which the sun begins to fall below the horizon

sunset (adjective)	time of day (adjective)
providing for termination
of a declining industry or technology