Michael A. Terrell writes:

Watches have a trimmer to adjust the frequency, but PC motherboards
don't, anymore. Some 286 and 386 motherboards had them, and could be set
to give fairly accurate time. I don't really care. I use software that
updates the clock from time to time and sets it to a few milliseconds
from the atomic clocks at NIST.

Even if you have a very accurate clock, you still need to synchronize
it with NTP or some similar utility if you are using the Internet.
Once you start communicating with other machines, it's vital that all
the machines be synchronized with respect to time of day--just having
a clock that accurately measures time isn't enough. So if you're on a
network, either you must synchronize your machine to the Net with
software, or you must have a hardware clock that synchronizes to some
external source, such as a GPS or radio-controlled clock.

In fact, if your computer is _not_ connected to any other computer,
then what you need is a clock that is very stable and accurate in its
measurement of time intervals. But if your computer is connected to
other computers, this stability and accuracy is far less important
than synchronization with the other computers.

If you have an isolated PC with a very accurate clock, you can set it
by hand, and perhaps it will be within one second of the correct time.
However, since the clock is accurate, you can be sure that it will
never be _more_ than one second away from the correct time. In other
words, the initial error is also the maximum error, and a one-second
error is often okay for a stand-alone PC as long as the error never
increases. Since the computer is not communicating with anything
else, the one-second error is not a problem.

If you have a PC connected to other PCs, the most important thing is
to have all PCs set to the same time. In theory, it doesn't even
matter if they drift, as long as they all stay locked together. So
clocks that are fast by a second a day are not a problem, as long as
all the clocks are off by the same amount. In the case of multiple
connected PCs, then, synchronization with each other takes priority
over long term stability with respect to the actual time of day.

Finally, if your PC is connected to the Internet, you need
synchronization with the "real" time of day, as maintained by atomic
clocks around the world. This ensures that your PC will have the same
time of day as all the other PCs on the network worldwide (assuming
they have accurate clocks). So you need software that synchronizes
your PC to the correct time of day. You don't actually need a very
accurate clock on the PC, though, because good synchronization
software--combined with a good operating system--will continually
"discipline" your local clock and ensure that the time of day on your
machine precisely matches the actual time of day worldwide.

In summary, if you have a PC that is continuously connected to the
Internet (broadband, for example), all you need is software that will
synchronize the clock regularly (and most operating systems have this
now--Windows XP does it automatically). If you have a PC that is
isolated and not connected to anything, _then_ you need either an
extremely accurate PC clock, or some external reference that you can
use to keep the clock on time, such as a radio-controlled clock, a GPS
clock ... or your own wris****ch.

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