You want to get this right once and for all.
No more getting confused deadline with dateline.
Deadline is when the time limit is up. It is the cut-off point or the date when you have to finish your thesis, project or manuscript. If you don't submit your project by the deadline, you're dead meat.
Dateline is a line at the head of a newspaper article showing the date and place of writing. If you miss it, so what? You can still read the news.