The Nuclear Doomsday Clock is a symbolic representation of how close the world is to catastrophic destruction by nuclear war. It is based on the real-world Doomsday Clock, which performs a similar function. The closer the clock stands to midnight, the nearer the world is to Armageddon.
The cover page of each chapter of Watchmen has an image of a clockface. The clock on the cover of Chapter I is set at twelve minutes to midnight, and on the cover of each subsequent chapter the clock is moved forward by one minute. The clock on the cover of the final chapter, Chapter XII, is set to midnight. The final page in each Chapter also bears a clockface showing the same time as the one on the cover.
Veidt's master plan takes place at midnight, New York local time. On page 27 of Chapter XI, a clock set to New York time is pictured at one minute to midnight. A clock, covered in blood and set to midnight, appears on the cover and first page of Chapter XII.
The Doomsday Clock makes a return appearance in DC Rebirth #1 and the eponymous series. At the end of DC Rebirth #1, the final two-page spread showed that Doctor Manhattan's experiments with time set the clock back to 11:45, fifteen minutes to midnight. It moves forward to twelve minutes by the events of Doomsday Clock. The clock takes on a new appearance for Doomsday Clock, with midnight represented by Superman's shield, once again appearing on the cover of each issue.
- Doomsday Clock at Wikipedia, the Free Encyclopedia