Re: Why is a computer's clock NEVER accurate ?

That depends on what you mean by "clock". My computer links via the internet 
directly to the Apple Americas/US (time.apple.com) server, which means the 
desktop clock on my computer is always accurate to the second. If you're 
referring to the CPU's clock speed (ie, my PPC chip, nominally clocked at 
266MHz), variations occur because even though chip production is a fairly 
precise art, it is not an exactly precise art. So even though a CPU may be 
clocked at, say, 400 MHz, it may actually run slightly slower or faster than 
that. 
 http://tycho.usno.navy.mil/what.html

also provides the exact time. Depending on your OS, you may have the option 
of connecting automatically to a clock server, thereby ensuring that your 
machine is always "on time". (Mac users: under the Apple menu, go to control 
panels>date and time, and enable the "Use a network time server" option).