Vintage watch timekeeping accuracy

While vintage watches might not have the tolerances of modern pieces with new technologies, it is reasonable to have them keep accurate time. They might not achieve the -4/+6 second average daily rate specification of a COSC (Contrôle Officiel Suisse des Chronomètres (COSC), the Official Swiss Chronometer Testing Institute) watch, but if it is of good quality and had been properly maintained, it is not unreasonable to have them running within 10 seconds each day or less.

Related, that when a watch is COSC-certified, it is certified at the point of testing (actually a period of testing), and once it leaves the factory, it no longer has any requirement to maintain that level of accuracy. But while you can’t expect a COSC-certified watch to maintain that level of accuracy throughout its life, it is possible that it may.