A period shall not end with the puck in either one of the end zones until it crosses the blue line into the neutral zone, unless it goes for icing, in which case the period continues.
A team with a power play that will not expire before the end of the period may elect to extend the period for the duration of the power play (if the first or second period is extended this way, the extra time is taken out of the next period; if it's the third period or an overtime period, the game is simply extended).