Watch function (also: eternal calendar, everlasting calendar; French: Quantième Perpétuel)
A complicated mechanism (complication) for large clocks and wristwatches, which displays the correct date of the Gregorian calendar up to the year 2100 without any external control intervention. It takes account of all the short and long months, of the twenty-ninth February, and of the leap years. The necessary control is performed in the movement by so-called program wheels.
With the so-called "semi-perpetual" calendar (also: annual calendar) the leap year, thus the 29th February in every four years, is not included.
For portable watches, there are two systems:
- the parallel running perpetual calendar: Here the passage of the hands a number to the other happens in two different speeds, at first slowly, then rapidly. In addition, date and day of week don't always jump exactly at the same time.
- the jumping perpetual calendar: here the days, months and date star is switched by a rocker in one quick motion at exactly the same time.
Wristwatches with perpetual calendars were manufactured by Patek Philippe as early as 1930 in single copies. From 1941 they were produced as standard by Valjoux on the basis of a 13-ligne ebauche movement. In 1950 also Audemars Piguet brought a wristwatch with perpetual calendar to market.
- Das große Uhrenlexikon; Autor: Fritz von Osterhausen; ISBN 3898804305