Time isn’t constant. It runs at slightly different speeds for everyone, depending on how fast you’re moving and how strong the gravity around you is. Albert Einstein figured this out over a hundred years ago, and we’ve confirmed it with experiments thousands of times since. It’s been the inspiration for movies like Interstellar.
The stronger the gravity around you, the slower your time passes compared to someone in weaker gravity. Near a black hole - where gravity is the strongest in the universe - time runs dramatically slower than out in deep space.
In theory, an astronaut who hovered close to a giant black hole’s event horizon for an hour could return to find that hundreds of years had passed in the rest of the universe. Their watch would show a single hour. Everyone they knew would be long gone. This is true to a tiny degree even on Earth: clocks at sea level run slightly slower than clocks on mountaintops. GPS satellites have to constantly adjust for it, or they’d drift off by miles.
