Atomic Clocks

No, you will not grow an extra limb if you touch one. These highly accurate clocks are the reason why we can use things such as GPS. Atomic clocks use oscillating masses like regular clocks, but the main difference is that the mass used in an atomic clock is an atom, hence the name. The oscillations of the atom are caused by the electrons around the nucleus.

Some elements used in Atomic clocks are Hydrogen, Rubidium, and more commonly, Cesium. Cesium clocks are made by heating Cesium so it passed through a magnetic field at a certain energy state. This energy state allows the atoms to oscillate at the correct frequency. And of course if the frequency is known, just do some simple arithmetic and you will have the time.

http://science.howstuffworks.com/atomic-clock.htm