This is incorrect. Clock frequency varies.
Many modern computers use an el.
Any Compaq ProLiant produced in the last 6 years keeps time to no better than plus or minus one to two minutes per day. Since these are generally used as servers, its a significant pain in the posterior - unless you are connected to the Internet, or have some other way to sync. In several stand alone systems I have had to use NTP to sync the Compaq machines to cheap no-name Taiwanese ones that are accurate to a few seconds per month. Silly, ain't it.