Chronometer

The deviations can be caused by external influences, such as changes in temperature or air pressure. The normal deviation of the accuracy is usually between 10 and 30 seconds/day. This value can drop to 6 seconds/day for high quality watches.
Even a watch with the title "official certified chronometer" has a small deviation despite many tests. As long as this is between + or - 4 to 6 seconds it can get a chronometer certificate.
A movement that deviates daily (86,400 seconds) by 30 seconds from the official time standard has an arithmetic error rate of 0.035%. Its degree of accuracy is already an amazing 99.965%. Chronometers can reach values as low as 0.005%.

Source Wikipedia: A watch can only call itself an official chronometer if its Swiss movement has passed a test (according to NIHS 95-11 / ISO 3159) by the independent Swiss observatory Contrôle officiel suisse des chronomètres (COSC). If the movement of a watch receives the COSC certificate, the manufacturer usually marks the watch with the word Chronometer. After passing the test, the movement receives a corresponding certificate attesting to its accuracy.