20.1. The System Time Module
All UNIX systems have an internal clock to keep track of the current time, even when the system is powered off. Linux systems effectively have two clocks—one that is maintained by the kernel when the system is running, and a separate hardware clock that runs all the time. The kernel's system time is set based on the hardware time when the kernel is loaded, so they should be synchronized. However, if one of the clocks is slower than the other it is possible for the hardware and system times to fall out of sync on a system that has been running for a long time.
All UNIX systems store the time internally as the number of seconds since January 1, 1970 GMT. For display, this is converted to a human-readable local time ...
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