There are only atomic clocks in some master servers: "TrueTime is implemented by a set of time master machines per datacenter and a timeslave daemon per machine

There are only atomic clocks in some master servers: "TrueTime is implemented by a set of time master machines per datacenter and a timeslave daemon per machine. The majority of masters have GPS receivers with dedicated antennas; these masters are separated physically to reduce the effects of antenna failures, radio interference, and spoofing. The remaining masters (which we refer to as Armageddon masters) are equipped with atomic clocks. An atomic clock is not that expensive: the cost of an Armageddon master is of the same order as that of a GPS master." The timeslave daemons running on each machine keep them synchronized with the master time servers, and maintain tight bounds on their inaccuracy. (Disclaimer: I work at Google)