HP48G Crystal / Timer Accuracy

[Steve Simpkin]

I used to have a small scheduled program on my HP-48SX that I wrote which would automatically run once every 3 days at 2:00 am. It would change the calculator time by a small fixed amount to correct the error drift of the crystal and then turn off the calculator. The result was that overall clock drift was less than a second a month making it the most accurate timepiece I owned (before radio controlled clocks). I determined the correction factor initially through measurement and then by making small adjustments. The effect on battery life was negligible. This sure beats opening the calculator and replacing the crystal. It should be more accurate as well.