Quinton is walking through the desert when he happens upon a mysterious statue. The statute contains a safe in its torso, guarded by two handles, one on the left side and one on the right side. The instructions on the statue read:

Turn the left handle. After exactly 18 minutes, turn the right handle. Untold wealth and riches will be yours.

Quinton lost his watch days before. Fortunately, the statue has two hourglasses in its hands: one marked 8 minutes, and the other marked 13 minutes. What is the easiest and most accurate way for Quinton to calculate exactly 18 minutes, and when should Quinton turn the handles?