NavList:

Greg Rudzinski, you wrote:
"When setting a Casio digital wrist watch I hack it to the exact tick by short wave"

This is what I was always taught, and I think most people do it this way. We even take a certain pride in hitting the button "right on the beep". Whether it's "right" or not depends on what we want the watch to read. Should it display the current time, to the nearest second, or should it display the last second that "ticked over". In the former case, the error in reading the time is -0.5 to +0.5 seconds with a mean error of 0.0 seconds. In the latter case, the error in reading is -1.0 to 0.0 seconds with a mean error of -0.5 seconds. It seems to me that when we do this by short wave or some other time source trustworthy to a tenth of a second, we should make an effort to start or set the watch about half a second before the tick. Then the mean error in the displayed time is zero.

Note that there are plenty of other errors to worry about when we get down to a single second. There's the reaction time error, which you alluded to. There's also the DUT correction --that fraction of a second that separates UTC (atomic time) from UT1 (earth rotation time). But none of those matter either if the watch is off by an average of 0.5 seconds...

Frank Reed