We change all batteries annually while we are doing the QA on them so it's not likely that a battery will fail during the year. However, things do happen and I agree with exlimely, if you do have to change or replace (say it fell out for some reason) a battery, then a 'calibration' would be prudent.
Also, we are not talking high precision here, i.e. wider acceptability range than a pipette would be, i.e. testing is done in ranges of time, not exact seconds. This 'calibration' is just a simple check up to make sure the timer isn't totally out of range. Most of the time, the error is the readout gets 'broken', not the accuracy of the timer.