dateline (n.)

also date-line, 1880 as an imaginary line down the Pacific Ocean on which the calendar day begins and ends, from date (n.1) + line (n.). Never set by any treaty or international organization, it is an informal construct meant to coincide with a line 180 degrees (12 hours) from Greenwich, but it always has followed a more or less crooked course.

Meaning "line of text that tells the date and place of origin of a newspaper, article, telegram, etc." is by 1888.