The Movie: A bunch of bureaucratic dummies shoot a nuclear missile at New York City, with a timer set to one minute. This gives Iron Man 60 seconds to intercept that sucker and chuck it into a wormhole.
Real Life: Nuclear missiles aren't time bombs. They are built to be explode above their target, causing an airburst that will wreak much more damage than a ground hit. It's much easier to just rig the device to an altimeter which will detonate the bomb once it reaches a certain height. Using a nuke as a time bomb would be a mathematical nightmare: the calculations would have to include the exact time a pilot released the missile, the air resistance, the atmospheric conditions, and a host of other variables including whether or not Iron Man is going to show up and throw the thing into a wormhole.