> There's a lot of focus in this thread on the atomic clocks but in most datacenters, they're not actually that important and I'm dubious that the hyperscalers actually maintain a "fleet" of them, in the sense that there are hundreds or thousands of these clocks in their datacenters.

I mean, fleets come in all sizes; but if you put one atomic reference in each AZ of each datacenter, there's a fleet. Maybe the references aren't great at distributing time, so you add a few NTP distributors per datacenter too and your fleet is a little bigger. Google's got 42 regions in GCP, so they've got a case for hundreds of machines for time (plus they've invested in spanner which has some pretty strict needs); other clouds are likely similar.