Question

What property of the pendulum makes it suitable for measuring time?

Answer 1

For a fixed-mass pendulum in a fixed gravity field, the period is constant. This means the device can be used to measure elapsed time. Just count the swings during the event you wish to measure, and you have the elapsed time accurate to one swing. The pendulum is an excellent way to measure time.

Answer 2

As it is used in clocks period of pendulum oscillations depend only on its length and strength of local gravity. So when you don't move your clocks it depend s only on length of pendulum.