Cesium reference clocks can operate with accuracy around 10^-14 (aka 0.01 parts per trillion). In a year, a cesium clock would slip by a few tenths of a microse

Cesium reference clocks can operate with accuracy around 10^-14 (aka 0.01 parts per trillion). In a year, a cesium clock would slip by a few tenths of a microsecond. That said, the whole "submerged computer must surface" thing is a bit of a red herring argument IMO. What use case would you have for needing to keep time in sync within seconds with the outside world, but being unable to communicate with that world? If you're trying to plan simultaneous delayed action across the world, it would suffice to merely be in sync with each other, leap seconds ignored.