The perpetual calendar is a watch complication consisting of displaying on a mechanical watch the day of the month (the date ), the month and the position of the year in the cycle of the leap years automatically: the change from the last day of the month to 1 st of the following month does not require any date correction by the user, including February 29 of a leap year .
Perpetual calendar watches were long regarded as masterpieces of watchmaking , so rare. There are now perpetual calendar watches of a more current production and at an affordable price.
The display is made possible by a needle or a disk that makes a turn in four years, and thus allows to identify the leap years every 4 years.
The Great Complication
There are some rare watches with a perpetual calendar taking into account the fact that the secular years are not leap-frogs, and some extremely rare models taking into account that only the secular multiple years of 400 are leap-frogs.
Since 1990 , the Swiss watch company International Watch Co. (IWC) produces the “Grande Complication”, a perpetual calendar watch that takes into account the secular non-leap years. The timing adjustment should only take place in February 2400, after which it will be correct until 2800. The watch has many other complications: lunar phase, minute repeat , etc. The watch has 659 pieces. To achieve the desired accuracy, a gear ratio of 1: 6,315,840,000 had to be achieved: in one century, the rocker made 25,228,800,000 alternations to move the secular slider by 1.2 mm .