System clock
The system clock is operated by the Linux kernel and updated through
interrupts.
The value of the system clock is the number of seconds since January first, 1970, 00:00:00. As the precision of this value is of infinite precision (if desired?), it is not limited to integers.
The system clock is initialized from the value of the hardware clock when the system is
booted up.
The current system clock time can be read with
timedatectl.