I'm thinking something like "X days hh:mm:ss remaining"
..changing from green to red as the deadline approaches
..changing to "expired" after the deadline, in case the puzzle closed while the player was away from the keyboard or had the window minimized.

I've seen veteran players say they didn't realize that a puzzle expired on a given day.
Some newer players have not yet translated 23:00 GMT to their local timezone.
Also some puzzles are set to expire at 23:55 GMT