round-the-clock vs nonstop

round-the-clock is not a noun while nonstop is a noun.

Word	Noun	Adjective	Verb	Adverb
round-the-clock	No	Yes	No	No
nonstop	Yes	Yes	No	Yes

As adjectives, round-the-clock and nonstop are synonyms defined as:

Other synonyms of round-the-clock include around-the-clock, day-and-night.

round-the-clock (noun)	nonstop (noun)
	a flight made without intermediate stops between source and destination

round-the-clock (adjective)	nonstop (adjective)
at all times	at all times
	(of a journey especially a flight) occurring without stops

round-the-clock (adverb)	nonstop (adverb)
	without stopping