In article , Isaac
Wingfield wrote:

You're very unlikely to be able to use any kind of frequency counter to
measure the oscillator with sufficient accuracy. There are close to a
hundred thousand seconds in a day. Measuring to one part in 10^5 will
leave you with an error of half a minute a month; not very good at all
for a timepiece these days.

I appreciate the comments. Now all I need to do is to look inside and see
if there is a trimmer. I can see where the slightest error would
accumulate over time.

I have an old Fluke counter (7220) with a low-frequency option. It
phase-locks a higher-frequency oscillator with the input signal, counts
the oscillator and displays it as the lower frequency. It would be worth
a try if I could pick up a strong enough signal from the watch without
warping it.

Fred