A device for measuring the accuracy of a mechanical watch includes a time reference source, a calculating mechanism, and another data processing unit. The time reference source is formed by an internal time reference source, or by a receiving and processing mechanism arranged to receive and process signals transmitted by an external time source to permanently update a repeater clock internal to the device. The calculating mechanism is arranged to calculate time differences between display values, and/or between time reference points provided by the time reference source, and to calculate a variation in rate. The another data processing unit is arranged to store and process at least one variation in rate calculated by the calculating mechanism, and/or a viewing mechanism arranged to display at least one variation in rate calculated by the calculating mechanism.