time of day vs hour

time of day and hour both are nouns.

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

As nouns, time of day and hour are synonyms defined as:

time of day (noun)	hour (noun)
clock time	distance measured by the time taken to cover it
	a period of time equal to 1/24th of a day
	clock time
	a special and memorable period