The short answer to this question is yes, they are accurate devices. However, a few different variables will come into play. One of which is human error but this will be touched on more in a little bit. The other is the accuracy of the device itself. As was mentioned earlier, you really should look for models that have been tested and calibrated for accuracy (within 1/100 of a second, ideally). But, at the end of the day, you and you alone will be required to start and stop the current time.