date-mark vs dateline

date-mark is not a noun while dateline is a noun.

Word	Noun	Adjective	Verb	Adverb
date-mark	No	No	Yes	No
dateline	Yes	No	Yes	No

As verbs, date-mark and dateline are synonyms defined as:

Other synonyms of date-mark include datemark.

date-mark (noun)	dateline (noun)
	a line at the beginning of a news article giving the date and place of origin of the news dispatch
	an imaginary line on the surface of the earth following (approximately) the 180th meridian

date-mark (verb)	dateline (verb)
mark with a date and place	mark with a date and place