Most clocks, from wristwatches to the systems that run GPS and the internet, work by tracking regular, repeating motions. To build a clock, you need something that ticks in a perfectly repeatable way.
A hobbyist who blogs as Chip Overclock wasn't satisfied with his GPS-disciplined desk clock. The Raspberry Pi inside it kept time within microseconds of UTC, but only when GPS satellites were ...
Time already behaves strangely in modern physics. It can stretch, slow, and split depending on speed and gravity.