1. The system clock
Timing by the Linux kernel by the working frequency of the CPU.
The date command query and set the system clock.
2. The hardware clock
The clock/hwclock command query and set the hareware clock.
3. How to synchronize the system clock with the hardware clock.
hwclock -s : Set the system time from the hardware Clock.
-w : Set the Hardware clock to the current system time.