nonstop vs round-the-clock

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

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

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

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

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

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

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