Gravity’s effect on time isn’t limited to intergalactic travel. Here on Earth, gravity can vary for several reasons, including your altitude, since you’re changing your distance from the centre of the Earth. That means if you put a bunch of synchronised atomic clocks at various altitudes, eventually those clocks would fall out of sync. A clock at the top of Mount Everest and one at sea level would, over the entire 4.5 billion-year history of the planet, have diverged by about a day and a half.