TrueTime is a highly available, distributed clock that is provided to applications on all Google servers.
Comparing to standard datetime libraries, instead of a particular timestamp, TrueTime’s now() function returns an interval of time [earliest, latest].
It also provides two functions:
after(t)returns true if t has definitely passed (e.g.t < now().earliest)before(t)returns true if t has definitely not arrived (e.g.t > now().latest)