At 04:35 PM 2/8/99 , Earl Truss wrote:
>I've been asked to explain why the current time shown in many applications
>is often so far off from a wall clock - often losing or gaining several
>seconds a day.  I was wondering if someone could point me in the direction
>for a fairly technical reference.

I don't have a ready technical reference but can give you my experience and
how I've dealt with it in the past.

I have found that the hardware clock (that runs when the computer is off)
is very inaccurate on may motherboards, but the software clock is very
accurate.  I have done experiments where I set the clock on a PC in the
BIOS then run the PC for a week.  The time is very accurate.  Then I set
the time in BIOS again and turn the PC off for a week.  On reboot the time
is off by several minutes in most cases.  I have done this on several PC's
and with different motherboards.  The amount of variation depends on the
particular motherboard clock. It seems that the clock interrupts, which
come from the on-board crystal for all the hardware timing, is accurate,
but the crystal which drives the "clock on a chip" while the computer is
off is not accurate.

Back in DOS days I had a program I used which could calculate the
inaccuracy of the PC hardware clock.  It would adjust the time each time I
booted up. It would calculate the + or - seconds to adjust the time
depending on the time that the PC was turned off. I had to manually adjust
the time a few times so that it could "learn" how many seconds per hour the
PC clock was off while powered down.  After that when I did have to
re-adjust the time, it would make more calculations to make it's
corrections even more accurate.  After a few weeks of learning, the PC's
time was very accurate.


Doug Simmons

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