A perpetual calendar (also: eternal calendar, everlasting calendar; French: Quantième Perpétuel) is 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 years with February 29, and of the leap years. The necessary control is performed in the movement by so-called program wheels.
In the Gregorian calendar the year 2100 is not a leap year. Some perpetual calendars (eg by IWC) support the consideration of this exception by a single operator intervention, though many perpetual calendars will require the user to advance the date on March 1, 2100. Many perpetual calendars will require a the year indicator to be replaced in future centuries and often ship with discs marked "21", "22", and so on.
Some perpetual calendar also include moon age and moon phase, a leap year indicator is possible, and other calendar complications. One unusual perpetual calendar watch is the IWC Portugieser Sidérale Scafusia which shows leap year and sequential day of the year but not day, date, or month.
For portable watches, there are two systems:
- Parallel perpetual calendar mechanisms run continually, advancing the calendar slowly before "jumping" at midnight, the first of the month, or new year. In these watches, the date and day of week don't always jump exactly at the same time.
- Jumping perpetual calendars switch the days, months and date at exactly the same time using a single star and rocker.
Patek Philippe is generally credited for bringing the perpetual calendar complication to the wrist, delivering such watches as early as 1930. Starting in 1941, Patek Philippe produced standard watches with a perpetual calendar on a Valjoux movement. In 1950, Audemars Piguet brought a wristwatch with their own perpetual calendar to market. The complication remained unusual through the following decades, though many watchmakers developed such movements.
All of these early perpetual calendars used correctors in the case to adjust the calendar if the watch was stopped. In 1985, IWC introduced a revolutionary new watch, the IWC Da Vinci Perpetual Calendar, which allowed all adjustment to be done using the crown. Developed by Kurt Klaus, this module was also the first to combine a perpetual calendar and automatic chronograph in the same watch. Later watches in this series added rattrapante and grand complication functions.
Today, most high-end watchmakers have their own perpetual calendar mechanism.
- Das große Uhrenlexikon; Autor: Fritz von Osterhausen; ISBN 3898804305