Should have used an alarm clock
Bond explains the reason that he has to detonate the giant bomb using a sniper rifle with explosive bullets is because if the police get wind of anything wrong, they immediately shut down all communications networks, which will interrupt a remote detonation signal.
Lets ignore the fact that all those people being in Times Square at the same time would cause total loss of cell coverage in that area anyway....
Why didn't he just use a clockwork timer?
HE WAS A BLOODY WATCH MAKER!!