Special Relativity: When you move a clock, it is slower as when it just sits there. When stopped, it runs with the normal speed again. (All relative to the non-moving observer.)
How's that? There are explanations out there, but wrote it down myself to understand it better.
For fun, I added a calculator to see how bad it can get. 4 hours in a plane and your clock lags ~4 nanoseconds (ignoring gravitation).
https://miamao.de/blog/2026-01/22.Moving_Clocks_In_Special_Relativity.html



