I hope the 6 hour limit is either not real, or something more interesting.

It may be that the limit is not due to the physics itself but because of the intervention of an early wizard. Unbounded time travel is one of the most powerful abilities imaginable. The first witch to exploit this and take ultimate power would obviously want to prevent others from overthrowing her. It would be in her best interest to find a way to put constraints on the powers of others. Longer than six hour anomalies may well trigger destructive countermeasures.