Perpetual calendar

A perpetual calendar is a date mechanism geared to the pattern of the Gregorian calendar rather than to a uniform 31-day month. Patek Philippe, describing its own calendar watches, says a perpetual calendar "automatically takes into account monthly variations as well as leap years", and that the firm built "the first wristwatch equipped with a perpetual calendar back in 1925, based on a movement we produced in 1898". That is Patek Philippe's own account of its own history.

The rule the mechanism has to follow comes from the calendar, not from the watch industry. The Astronomical Applications Department of the US Naval Observatory states it as follows: "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is."

Ordinary perpetual calendars implement only the first part of that rule. A patent application for a secular perpetual calendar, filed by Franck Mueller Watchland SA with Pierre-Michel Golay named as inventor (priority 14 February 2006), states that "the correction made every four years by a conventional perpetual calendar is too great according to the Gregorian calendar", because "on each intermediate century, 2100, 2200, 2300 and 2500, 2600, 2700, etc., the leap year is eliminated". The consequence follows arithmetically from the two sources: 2100 is divisible by four but not by 400, so a mechanism built on the four-year cycle alone will show 29 February 2100 and need correcting by hand.