Today I learned that clocks on GPS satellites are set to tick slightly slower than those on the surface of Earth because the gravity at their altitude is less – and gravity affects time.

Here is from the article Star Eater in National Geographic of March 2014, page 102:

Time and black holes have a very strange relationship. Actually time itself — forgetting about black holes for a moment — is an unusual concept. You probably know the phrase “time is relative.” What this means is that time doesn’t move at the same speed for everybody. Time, as Einstein discovered, is affected by gravity. If you place extremely accurate clocks on every floor of a skyscraper, they will all tick at different rates. The clocks on the lower floors — closer to the center of the Earth, where gravity is stronger — will tick a little slower than the ones on the top floors. You never notice this because the variances are fantastically small, a spare billionth of a second here and there. Clocks on global positioning satellites have to be set to tick slightly slower than those on Earth’s surface. If they didn’t, GPS wouldn’t be accurate.

Black holes, with their incredible gravitational pull, are basically time machines. Get on a rocket, travel to Sgr A* [a famous black hole]. Ease extremely close to  the event horizon, but don’t cross it. For every minute you spend there, a thousand years will pass on Earth. It’s hard to believe, but that’s what happens. Gravity trumps time.

So by orbiting a black hole for a few minutes, you can travel thousands of years into the future. Where do I sign up?