why is my clock on my macbook running increasingly fast?

Can anyone tell me why the clock on my computer (Macbook Pro w/ retina) is gradually starting to run faster? It used to be exactly in sync with my iphone, but I noticed maybe like 5-6 months ago that it was running a couple minutes faster, and over that time it has slowly but steadily sped up even more, to where it is now nearly 8 minutes fast. It's certainly a minor problem, it's not so much that it bothers me as that i am just curious, and also would want to know if it were indicative of some other kind of problem with the computer. Thanks 🙂