Google's turned on a set of public network time protocol (NTP) servers.
You'll find the servers at time.google.com – which resolves to 216.239.35.0, a rather less pretty IP address than the 8.8.8.8 and 8.8.4.4 Google uses for its public domain-name system (DNS) servers.
There's also time2.google.com at 216.239.35.4, time3. at .8 and time4. at .12.