That saying is wrong. No timepiece keeps a perfect rate, therefore nobody knows exactly what time it is. The closest the average person can get is to listen via shortwave to the WWV time signal, but you'd better apply a correction to allow for the time it takes for the radio signal to propogate from the transmitter to your reciever, and then another correction for the distance between the radio's speaker and your ear (unless you're watching the signal on a 'scope).
Originally Posted by fotch
A watch or other timepiece that has stopped is at least correct twice per day (assuming it has a 12hr dial, once per day if a 24hr dial) but you'll never know precisely when.