====== Perpetual calendar ======
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A **perpetual calendar** is a [[watch]] [[complication]] that maintains the calendar over leap years.

===== Background =====
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 year]]s. The necessary control is performed in the [[movement]] by so-called program wheels.

This contrasts with an [[annual calendar]] which adjusts to short and long months but not leap years and a [[simple calendar]] which assumes that all months have 31 days.

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.

===== History =====
Perpetual calendars has already been developed for [[astronomical watch|astronomical]] clocks, notably in [[Prague]] and [[Geneva]] as early as the 17th Century.

[[Louis Elisée Piguet]] is said to have produced the first perpetual calendar mechanism for pocket watches in the 19th century. His family firm, [[Frédéric Piguet]], would produce these modules for other high-end brands through the 20th century and continues to produce them today as [[Manufacture Blancpain]].

[[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 [[corrector]]s 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.

===== See Also =====
  * [[Annual calendar]]
  * [[Secular perpetual calendar]]

===== Literature =====
  *Das große Uhrenlexikon; Autor Fritz von Osterhausen; ISBN 3898804305


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