ABSTRACT
We take for granted that computers on a network have some means to set their clocks to the nominal time of day, even if those means amount to eyeball and wristwatch. How accurately can this be done in practice? Most folks who have to get to work on time set their wristwatch within a minute or two of radio or TV time and expect it to drift less than a minute over the month. This amounts to a rate error of about 23 parts per million (ppm), not bad for a temperature-stabilized wrist. Real computer clocks can be set by wristwatch usually to within a minute or two, but some have rate errors ten times wristwatch. Nevertheless, in many applications, the accuracy maintainable by a herd of wristwatch-wrangled network timekeepers might well be acceptable.
