call it a day vs call it quits

call it a day and call it quits both are verbs.

Word	Noun	Adjective	Verb	Adverb
call it a day	No	No	Yes	No
call it quits	No	No	Yes	No

As verbs, call it a day and call it quits are synonyms defined as:

call it a day (verb)	call it quits (verb)
stop doing what one is doing	stop doing what one is doing