This is another question that requires general relativity to answer. The short of it is that something which is in a gravity well will experience time dilation relative to something outside of that well. This is actually needed for GPS to work accurately: here on Earth time passes ever-so-slightly more slowly than it does for the satellites in orbit. Again, you need some pretty advanced math in order to even touch general relativity quantitatively, which I as a biologist never was exposed to.