How to Synchronize Your Clocks to a High Degree of Accuracy

This basic model actually assumes that there is a well-defined notion of what it means to be instantaneously at the same time, which isn't exactly true because of relativity and stuff like that. So where does that external reference come from? I'm not an expert on this stuff, but I'll give you this sort of 10,000 foot view. You have various physics laboratories all over the world, like NPL in the UK and other places across the world. They all have measurements of what they think time is using things like hydrogen measures and sort of very accurate atomic methods.