The Doomsday Clock is a public relations device created by the Bulletin of the Atomic Scientists. It was first created in 1947 as a means of illustrating dramatically the views of the journal's editorial board (made up largely of Manhattan Project scientists) with regards to the probability of world affairs going in a dramatically disastrous direction. It has gone up and down over the years. You can read the ups and downs here. It operates by the journals' editorial board getting together every year or so and deciding whether they should move it up or down. It is not a "real" clock.