When it was created by the magazine's staff in 1947, it was initially set at seven minutes to midnight and has moved 17 times since then.
It was as close as two minutes to midnight in 1953 following U.S. and Soviet hydrogen bomb tests, and as far away as 17 minutes to midnight in 1991 after the superpowers reached agreement on a nuclear arms reductions.
Since this back-and-forth indicator doesn't seem to behave much like a clock, I'm not sure these guys are using the right model. I think I'd have chosen a thermometer.
Or maybe a 5 level color-coded indicator that's always yellow unless we're in a nuclear war.