Rolex calibers made in-house are considered to be especially precise. Each watch comes with a certificate from the Official Swiss Chronometer Testing Institute (COSC). The COSC measures the precision of a movement in five different positions and three different temperatures based on a standardized testing method. The entire testing process lasts 15 days. Rolex founder Wilsdorf also valued precision back at the beginning of the 20th century. In 1910, he sent a watch to a local watchmaking school in Biel so it could be evaluated. The movement was subjected to a 14-day test, which it passed. At the end of this two-week procedure, the testers issued the first wristwatch chronometer certificate worldwide and sent it to London.
The next sensation followed only four years later: Wilsdorf sent an Aegler movement to the National Physical Laboratory in London. The caliber passed their tests with flying colors. It was subjected to three different temperature conditions and five different positions: crown facing up, left, and right, and dial face down and face up. After 45 days, the watch, with its golden case and blue enamel bezel, was named the first wristwatch in history to receive a class A certification. This type of certification was previously only awarded to larger watches, such as navy chronometers.