date line vs dateline

date line and dateline both are nouns.

Word	Noun	Adjective	Verb	Adverb
date line	Yes	No	No	No
dateline	Yes	No	Yes	No

As nouns, date line and dateline are synonyms defined as:

date line and dateline: an imaginary line on the surface of the earth following (approximately) the 180th meridian

Other synonyms of date line include International Date Line.

date line (noun)	dateline (noun)
an imaginary line on the surface of the earth following (approximately) the 180th meridian	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 line (verb)	dateline (verb)
	mark with a date and place