Time zones screw up Conal's plot somewhat. If the final commercial goes off simultaneously, then it'll be midnight on the East Coast, and lots of his targets will be in bed. If it goes off at 9 on each time zone, then someone besides Challis should have managed to make the connection between the commercial and the masks — maybe not by Central Time, but almost certainly by Mountain Time. The ending scene would have taken place long before Challis escapes Cochran's factory. Still, though, Cochran's guaranteed to massacre the children of at least one time zone.
But the only reason Challis knows exactly what's going on with the masks and the signal is because Cochran's told him. Any parents in other time zones would probably be too frantic and terrified trying to save their children to put two and two together, especially when it comes to something as seemingly innocuous as a television commercial that they've seen about two million times across the past few days. Also, as the Eighties Dan episode dealing with the movie suggests - we only see the adverts in one time zone; the others could have been at five, six, seven, or eight dependent upon where they are, and each time zone could have descended into general chaos with no-one thinking or knowing to call other TV stations in other time zones.