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1.
The clock that shows how close humanity is to "Midnight", a nuclear holocaust. As of 2002, the clock sits at 7 minutes to midnight.
In 1984, the doomsday clock was at 3 minutes to midnight.
by Gann May 29, 2006
24 10
 
2.
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.
John started keeping time on the doomsday clock after his boss had told him he'd be fired if he hadn't finished all of his paperwork before 6pm on Friday.

Or

For dramatic purposes whenever you see a bomb with a doomsday clock on TV or in the movies it always stops with just one second to spare.
by The Captive Spirit September 06, 2010
2 3