[Bfd70]

... the Contrôle Officiel Suisse des Chronomtres (COSC) standard deviation rate of -4 to +6 seconds within a 24 hour day, tested over a ten day period of chronometer testing, in five positions and at three temperatures. Watches that are within -4 to +6 seconds deviation in a 24 hour period are awarded Official Chronometer Certification by COSC and the movement is engraved with a chronometer serial number. Rolex has a self-imposed stricter standard of -1 to +5 seconds per day and hence the designation, "Superlative Chronometer Officially Certified" on the dial of any certified Rolex watch