Clocks

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/