Chronometer

A chronometer is a watch whose movement has passed an official precision test. In Switzerland, the title is certified under ISO 3159, which requires a mechanical movement to keep an average daily rate between minus four and plus six seconds a day.^[1]

The testing covers the movement in several positions and at different temperatures over a period of days.^[1] A chronometer certificate tells a buyer the movement met the standard when tested, not that the watch still does after years of wear.

Some brands add their own, stricter testing on top. A certified watch that runs poorly today still needs regulating or servicing, which is worth checking before you buy.