Tuesday, January 10, 2012 The official period clock starts on the first whistle of the first jam.

This one is simple. The period clock does not start until the game starts. The first (jam start) whistle of the first jam of each period is what starts the period. Thus, when the period starts, the period clock starts. Although the period clock may stop at points throughout the period, it starts initially when the first jam of each period starts.