Question

Imagine a grandfather clock. When the clock swings all the way to the right it 'ticks', when it swings all the way to the left it 'rocks'. If you wanted the time between the tocks and ticks, what length should the pendulum be? And, if you took the pendulum to the moon where g=1.67 m/s2, what would the time interval be? (I understand the concept and the givens, but I don't know how to calculate it!