w_tom wrote:

Watches have an internal capacitor to adjust for each
crystal. I have never seen that capacitor on motherboards
since (I believe it was) the IBM AT. Furthermore, the PC
clock operates at two significantly different voltages that
will change crystal frequency. Battery voltage and voltage
when PC is powered will cause additional fluctuation. Which
voltage should they adjust the capacitor to? Just easier to
not install and adjust the capacitor.

I have an ancient Seiko quartz with a trimmer inside, and by adjusting
it I was able to make it accurate to 30 seconds a year. But few cheap
watches have them, including none of those I tried in this test.