The modern approach is common view time transfer. Both clocks can observe signal sources such as satellites and compute their relative offsets. The signal do

The modern approach is common view time transfer. Both clocks can observe signal sources such as satellites and compute their relative offsets. The signal doesn't need to be particularly accurate (though it doesn't hurt), though you do need a good model of its position and propagation. GNSS satellites are an obvious choice though there are also specialized services for CVTT. Note that this is distinct from syncing from GPS, which is a thing people obviously do too, but CVTT can achieve much higher accuracy. Because you're synchronizing extremely stable clocks the difference between them will primary be an offset (plus/minus a slope from relativistic effects of different altitude). Because of this you can average a large number of readings, so the only major source of error will be systematic effects in propagation/orbit/etc. Historically, sync was obtained via traveling clocks-- e.g. you sync one atomic clock up and load it, running, in a station wagon... which is the same thing that is most often done for voltage standards today (as atomic voltage standards remain rare, compared to atomic clocks-- I think the least I've paid for one is $15 excluding the ones that were free). But vibration isn't great for anything with a crystal oscillator in it, and the most modern atomic fountain clocks don't work if they're accelerating in any direction except the designed 'up' direction (gravity), because the little cloud of cooled atoms will fall out of the measurement channel, which makes sync by station wagon not viable. Of course, once you talk about syncing there is always a question of what you're syncing to. UTC doesn't exist until after the fact. Laboratories measure their offsets via CVTT and UTC is calculated after the fact as past offsets to each of the contributing clocks.