Message from C, C++ discussions
This is from the specs of a database management system I am working on, note what time may look like in a distributed system
The time is directed by the coordination server. Each server must synchronize to that time.
The time is never guaranteed to be the real time, but it is guaranteed that any time query return value will be strictly greater than the highest recorded time of the server. When the system is running in normal and degraded state, the unit of the subtraction of timestamps is nanoseconds, else it is undefined.