Also, 
man hwclock 
has some good information on System Time versus Hardware clock time. I
think, to this day, hardware clocks are still inaccurate (good to only
so many parts per million). 

On Thu, 2003-10-16 at 11:00, Jeremy C. Reed wrote:
> On Thu, 16 Oct 2003, der.hans wrote:
> 
> > > hwclock --localtime --systohwclock
> > >
> > > Now the clock seems to know it's running in local time. I'm still
> > > curious, however, about the whole notion of system time being separate
> > > from the hardware clock time. AFAICT, Linux is the only OS that makes
> > > the distinction. I can't quite understand what the distinction is for.
> >
> > UNIX makes the disctinction. The others are incorrect.
> >
> > The hardware clock should be constant and should be GMT ( or whatever it's
> 
> I think Chris is talking about the difference between the hardware
> (CMOS, BIOS) clock and Linux's own system clock.
> 
> I believe the reason is because the Linux kernel was developed on hardware
> that had inaccurate hardware clocks. The Linux kernel can keep time better
> than the hardware clock itself.
> 
> Have a look at the Clock Mini-HOWTO.
> 
>    The Real-Time-Clock (RTC) chips used on PC motherboards are notoriously
>    inaccurate, usually gaining or losing the same amount of time each day.
>    Linux provides a simple way to correct for this in software, which can
>    make the clock *very* accurate, even without an external time source.
> 
> http://www.tldp.org/HOWTO/Clock.html
> 
> I was trying to find references to this in the kernel source .. but
> haven't found yet.
> 
>    Jeremy C. Reed
>    http://bsd.reedmedia.net/
> 
> 
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