The chronometer is an extremely accurate watch and is a highly sought after feature for watch enthusiasts. Only a small percentage of watches have this certification, which is not easily earned.
So how do you tell if a watch is a chronometer?
In order to be classified as a chronometer, a watch has to pass a rigorous array of precision testing. Over a 15-day period, the Official Swiss Chronometer Testing Institute (COSC) tests the movement of the watch to make sure it adheres to strict accuracy standards. A chronometer watch must keep time, on average, within -4/+6 or 10 seconds a day.
Chronometers will be marked as "Certified Chronometer" or "Officially Certified" on the dial and the serial number for the individual movement officially recorded.