The Express time clock is as accurate as the component that is used on the circuit board. The Crystal located at [Y4] on the 4110B5001 Express processor PCB is accurate to +/- 60 parts per million. ETC part number [Y150, Crystal 32.768 Khz]
This means that the Express product lines will gain or lose +/- 2 min per month or +/- approx. 5 seconds per day the specification.