The clock represents the threat of nuclear annihilation. It was created by the board of directors of Bulletin of the Atomic Scientists and first appeared on the cover of that magazine in 1947—two years after the United States had used two nuclear weapons against Japan (at Hiroshima and Nagasaki) to end World War II (1939–45). The atomic scientists developed the idea in order to illustrate the threat of total destruction posed by nuclear weapons. On the clock, midnight is the time of destruction. When the clock first appeared, the scientists had set the time at seven minutes before midnight. In the decades since, the clock had been adjusted based on the proliferation of or agreements to limit nuclear weapons.

The closest it ever came to “doomsday” was two minutes until midnight. This was in 1953, shortly after the United States and the Soviet Union each tested hydrogen bombs. The farthest the minute hand has ever been from striking the hour of midnight was in 1991, when the United States and the Soviet Union signed the Strategic Arms Reduction Treaty (START) and announced cuts in nuclear weapons. The scientists moved the clock to read 17 minutes until midnight.

In the late 1990s the clock read 14 minutes to midnight, but the 1998 testing of nuclear weapons in Pakistan and India, neighboring countries long at odds with each other, resulted in the clock being forwarded to nine minutes before midnight. In 2002 the Bulletin of Atomic Scientists informed the world that the clock had been adjusted to seven minutes to midnight, saying that not only had little progress been made on global nuclear disarmament, but the United States had rejected a series of arms control treaties and announced that it would withdraw from the Anti-Ballistic Missile Treaty. Further, terrorists sought to acquire and use nuclear and biological weapons. All of this added up to a greater threat of nuclear annihilation.