When the pitcher steps on the rubber would be a better starting point for the start of the play. Many plays don't even involve a pitch, and wouldn't be counted in your definition.
That's a great point and I agree with you. However by that logic, shouldn't the amount of time counted for football include when the quarterback comes under center? There's a lot of football going on when Peyton Manning comes to the line of scrimmage. Or when a quarterback is trying to draw the defense offsides.
Pick-off moves were the first thing that came to my mind.