an imaginary line on the surface of the earth following (approximately) the 180th meridian
"The ship's captain adjusted their course to cross the international dateline, knowing they would officially enter a new calendar day once they passed it."
a line at the beginning of a news article giving the date and place of origin of the news dispatch
"The editor told me to add the dateline to the top of the report so readers know exactly where the story came from."
A line at the beginning of a document (such as a newspaper article) stating the place of origin and typically the date, and often written in capital letters.
"The editor added the dateline to the top of the report to show that the story came from Tokyo yesterday."
To attach a dateline to a particular document
"The editor asked me to add the dateline to the press release before we sent it out."