Well, it's dated against pulsars and stars. But those sources of information have a bit of an error bar on time-space distance. Which is why a synthetic clock

Well, it's dated against pulsars and stars. But those sources of information have a bit of an error bar on time-space distance. Which is why a synthetic clock is needed here. That will have a known inception date and the changes if any can be compared. The problem with both is they're not exactly fully closed systems anyway so there will be some margin of error ever with the length of the operation. And during the test, we might just find out something completely unaccounted for in current physics... That isn't a universal constant related at all.