When your computer is turned off, the hardware clock — a.k.a. the CMOS clock — keeps the time. In contrast, when Linux is running the system time — the time kept by the kernel — is the definitive time source. System time is more accurate than CMOS time. It’s the time provided by the date command, and it is measured as the number of seconds since 00:00:00 January 1, 1970 UTC. The only way to keep correct UTC time on your computer is to synchronize with an external time source.
Link: Linux.com
