We offer two different options for timer calibrations: an outside lab, (Transcat) and in-house. There are some difference to know about.
Our outside lab uses the time-base method, of certification, which means they put the timer on a machine to detect the oscillation frequency of the vibrating crystal. This method is very quick, however it only check the crystal and not the rest of the circuit. (page So an overlooked fact is that you really need to do some sort of direct comparision in addtion. Our outside lab does not do this double check. (see page 53 7.B.4 of the NIST Practice Guide)
In comparison, we use the Photo Direct Comparison method. (description in section 6.C of the NIST Practice Guide) Essentially we are comparing the result of the timer under test to a NIST certified timers. In order to eliminate operator timing errors in starting and stopping the device, we use a photo to compare the times. As long as the photo accurately captures the digit segments (and it does) this method produces an accuracy equal to the time base method.