Why is it that a pendulum's period will gradually increase at first and then gradually decrease to its initial value?

What gradually increase
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I thing pendulum's period does not change with time

This will occur with the period if the pendulum is filled with water and gradually begins to leak until empty. It is about how the center of mass of the object changes.

The time for one pendulum would always be constant as gravity acts on it. The distance the pendulum moves from side to side decreases, the pendulum slows down but the time of one oscillation would always remain constant and the same to the first oscillation.

Yeah, clearly a normal pendulum's period is not a function of time.