[time-nuts] Windows XP time

Bill Hawkins bill at iaxs.net
Wed May 17 17:06:12 EDT 2006

Joseph Gray wrote:
"I knew SNTP was not as good as NPT, but I would have thought that SNTP kept
the clock accurate to at least the nearest second. Obviously not. So, what's
the worst that a typical PC clock would be off when using SNTP?"

Truly, the answer is, "It depends."

SNTP is designed to transfer the time from one machine to
another with compensation for network travel time. It is
precise if the network delay is symmetrical - same in both
directions. This is seldom true. Even if the delay was
symmetrical, your PC works with priority interrupts. A higher
priority interrupt will destroy symmetrical network delay.

The other source of error is the drift in your PC clock.
NTP adjusts the clock frequency if you have an adjustable
clock frequency, as is found in Unix derivatives. AFIK, a
wintel clock frequency can't be adjusted. So the PC error
depends on the frequency error and the rate at which the
PC requests an SNTP update. I've found the Mount Hay Tech
server to be able to keep a Win98 PC well within a second
with hourly updates. YMMV.

Bill Hawkins

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