The doomsday clock is a symbolic clock face managed by the Bulletin of Atomic Scientists. Initially started in 1947 it was until recently a direct reflection of the threat of Global Nuclear Conflict however more recently it has taken into account the threat of regional nuclear conflict and climate change.
It started in 1947 at 11:53pm or 7 minutes to midnight, with midnight being “all out war”. Over the next few decades it got closer and further away as global tensions changed. Its closest time being 11:58pm or 2 minutes to midnight in 1958 when the US & USSR tested thermonuclear devices within 9 months of each other. It reached 11:57pm in 1984 following the introduction of GLCM’s into europe.
Its farthest time was in 1991 when it was wound back to 11:43pm following the START and INF treaties.
It is worth noting that the clock is not an immediate response so was unaffected for example by the Cuban Missile Crisis as it was over in a short period of time.
The current setting is 11:55pm. 5 minutes to midnight. (confirmed as of 31st Oct 2013)
More information can be found at the Bulletin of Atomic Scientists website.