The short answer
If your vintage watch has a quickset date, move the hands into the lower half of the dial before using it. As a simple habit, we recommend positioning the hands around 6 o'clock before changing the date.
Avoid using the quickset while the watch is approaching or passing through its normal date-change period.
From the bench
A vintage quickset is a mechanical shortcut. It allows you to move the calendar independently rather than advancing the hands through repeated 24-hour cycles.
The important thing to understand is that the watch also has to change the date automatically once every 24 hours.
As midnight approaches, parts of the movement begin engaging with the calendar mechanism in preparation for that change. Depending on the movement, this process can begin several hours before midnight and remain engaged for some time afterwards.
Trying to operate the quickset while those components are already engaged can make two mechanisms attempt to act on the calendar at the same time. On some vintage movements that can damage teeth, intermediate wheels or other parts of the calendar mechanism.
What we recommend
Don't try to remember the precise danger period for every calibre you own.
Instead, develop one simple habit: before using the quickset, move the hands to approximately 6 o'clock. Once the hands are safely in the lower half of the dial, set the date and then return the hands to the correct time.
It takes only a few seconds and removes the guesswork.
If you're unsure whether a particular vintage watch has a quickset, or its setting mechanism feels unusually stiff, don't force it. Different movements use different systems, and with a vintage watch resistance is usually a good reason to stop and find out why.
A little mechanical sympathy goes a long way.