system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is time measured in space?

On earth time is measured via its orbit. But what about when you are not on a planet?
gpt
Traditionally, time is measured in years (the Earth orbiting around the sun) and days (the Earth spinning away from the sun and back). However, these durations are for our convenience and have no real, universal significance. Astronauts living in space still use days. For instance, on the International Space Station, the astronauts use a day/night cycle that's based on the time in England (GMT).

Now, if your question is how to MEASURE time, that's fuzzier, especially if you get into relativity. The easiest way is to keep around a device that measures periods of a cesium 133 atom. Every 9,192,631,770 periods is one second.