A Swiss watch may only be called a chronometer if its Swiss movement has passed a test in accordance with NIHS 95-11/ISO 3159 conducted by the independent Swiss observatory Contrôle Officiel Suisse des Chronomètres (COSC).
The chronometer test at the COSC takes 15 days. All tests take place at approx. 24 per cent humidity. After every 24 hours, the deviation is measured, and the movements are wound and readjusted. On the tenth day of the test, any complications such as chronographs etc. are switched on to determine their effect on the rate accuracy. The rate of the movement is determined in five different positions at three temperatures.
After passing the test, the movement is issued with a certificate confirming its accuracy as a chronometer. The movement is identified by an engraved number and a COSC certification number.