Why is the exact time of performance not available for some performances?

The exact time (hour) of performance is only available one month prior to the performance. However, the operas and musicals in Vienna at start between 16:00 and 20:00 (4:00 PM and 8:00 PM). The begin hour of the performance is also printed on the ticket. The exact begin hours will also be available in our website few weeks before the performance.