What is in the past is already destroyed.
What is in the future has not yet arrived.
What is in the present does not abide.
Although we contemplate in this manner the production, destruction, scattering, and demolition of mental phenomena and phenomena arising in the mind, we nonetheless remain constant in the accumulation of the roots of virtue and wisdom leading to enlightenment.
This is what is meant by skillful means in contemplating the three periods of time.