Pendulum Trap

Congratulations, you took the path to the left and arrived at the dwarf island bear cave 18 minutes before the poachers! You found bear tracks outside the cave, but no bears in the entrance. You need time to explore the cave

Fortunately, there is a large, sturdy tree directly above the cave and a large boulder off to the side up on the hill.  You tie a rope to the boulder and the tree and wait for the poachers. When they approach the cave, you will release the boulder and protect the bears, but the timing must be perfect. How many seconds before they reach the cave entrance should you release the boulder?

Hint: The time for a complete swing of a pendulum is given by T below.  L is the length of the pendulum and g is the gravitational acceleration, which for earth is 9.8 m/s².