The pendulum on your clock functions like the cruise control on a car. The longer the pendulum is, the slower it will run.
To adjust the pendulum, look under the pendulum bob (the bob is the shiny disk on the bottom of the pendulum) and you will see screw threads and a fancy nut (it’s called the rating nut). If you turn the rating nut from the left to the right, the bob will go up, and the clock will go faster. If you adjust the rating nut clockwise, the bob will be lower, and the clock will run slower. On a grandfather clock, a full turn of the rating nut will make a ½ minute a day change. If your pendulum is less than a foot long, a full turn of the rating nut can make a 1 minute a day difference.