Ewiger Kalender

The first watches with a perpetual calendar were produced as early as 1920. A so-called "perpetual calendar", is a complicated additional device in mechanical watches to display the date, day of the week, month, year and moon phase, taking into account leap years. 

A very important note for new owners of a perpetual calendar watch is that the watch must not be adjusted under any circumstances between 23.00 at night and 01.00 in the morning. In most cases, the movement will already start to change the date here so that the process is completed after 0.00 o'clock. To avoid any problems, the watch should be set to not reverse and the date display should ideally be set between 06.00 and 18.00 hours. 
Under no circumstances should the date be set backwards, as this will most likely destroy the mechanism.

The only exceptions to this are currently Ulysse Nardin and H. Moser & Cie. 
Watches with a perpetual calendar are only found in the upper price segment due to their small production quantities and the extensive design and manufacturing costs. This function is therefore one of the three "grand complications" (the others being the minute repeater and the tourbillon). Not many watch manufacturers are able to incorporate such complicated functions in their watches, including Patek Philippe, Vacheron Constantin, IWC and Jaeger-LeCoultre.
 
Here are a few models, each of which belongs to the top class of its manufacture: