Cade Metz, in Wired, on Google, leveraging with the Earth's gravitational spin:
These time-keeping devices connect to a certain number of master servers, and the master servers shuttle time readings to other machines running across the Google network. Basically, each machine on the network runs a daemon — a background software process — that is constantly checking with masters in the same data center and in other Google data centers, trying to reach a consensus on what time it is. In this way, machines across the Google network can come pretty close to running a common clock.
It's all GPS, atomic clocks, and NoSQL.