While a cesium clock may be off one second in 100 million years, the optical clocks will take 15 billion years to have the same magnitude of inaccuracy. If you want to build your own clock ...

Maybe that’s even more than half of the fun. [CuriousMarc] got an HP 5061A Cesium clock, a somewhat famous instrument as the model that attempted to prove the theory of relativity. The reason?