They use portable clocks: The US national reference and the portable reference difference was this when it left, and this when it came back and in France, the diff was this. When the Finnish clock was in France, the results were this and when it was in UK, results were this. When the UK reference was in US, it showed this, in France this, and In Germany, this. And so on, you get the idea. After that, it is statistics and math.
I have some stories about Mr. Clock and his adventures in first class air travel, like if he got served food and how the dessert was divided between him and his assistant, does every passenger with a seat reservation really need to have a passport as well and so on.