The clock that shows how close humanity is to "Midnight", a nuclear holocaust. As of 2002, the clock sits at 7 minutes to midnight.

A timepiece that counts down to the exact time of a deadline, failing to complete a certain task by the end of that deadline will result in dire consequences.

May refer to a metaphorical doomsday clock i.e. watching an actual clock and doing the math to see how much time you have left.

Or a physical doomsday clock like the ones that were sold in the lead up to the millennium or the ones featured on bombs that explode once they reach zero.