The explosion of a single atom bomb, while massively catastrophic, wouldn't be "doomsday" in the same sense that a "use 'em or lose 'em" launch of a superpower's entire arsenal of ICBM's would be. Assuming that's the sort of Doomsday the Clock was meant to dramatize and warn against, we're, if anything, farther from the brink than we were in Cold War days.