The first missile? When it hits Baghdad or when it is fired?
When the first bomb dropped or the time the bomber takes off?
If any of the military currently in the region are part of the attack, then the attack has already begun.
I'm assuming that President Bush will announce the official time when he addresses the nation to say that the attack has begun.
That man knows how to run a country and a military operation. I bet that the operation commanders love to work under his direction over the bent one's operatives.