what time it is.
Typically the clock source is a monotonic, atomic counter which will provide
-n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over.
+n bits which count from 0 to (2^n)-1 and then wraps around to 0 and start over.
It will ideally NEVER stop ticking as long as the system is running. It
may stop during system suspend.