It's not impossible at all, you just need to agree on a reference frame. Which is not hard to do when you're sitting on a planet.

And with computers it's fussy to get clocks synchronized below a millisecond but not that fussy. You can keep a single computer synchronized down to a single cycle, making timestamps effectively perfect across different parts of it. And then between computers, if you have n latency between them you should be able to get the clocks within n/10, which might as well be perfect for coordination purposes.