One of the most sought-after complications on today’s watch market—both in vintage and in new timepieces—is the perpetual calendar, which offers a host of functions for the hyperscheduled, in-demand modern wearer. These sophisticated time trackers typically hold anywhere from 300 to 600 pieces in their complex movements and offer indications that are accurate for hundreds of years. While they command prices ranging from $18,000 to well into the six figures, depending on their added functions and adornments, these watches can have waiting lists of up to several years because they are so complicated to build.

Essentially, perpetual calendar watches track and display, automatically, the day, month, and date. They are self-adjusting to compensate for short months and leap years. With the exception of a few that will run for centuries, today’s perpetual calendar watches typically are built to track time until the year 2100 before needing an adjustment.

Indeed, perpetual calendar watches house extraordinarily complex calibers. These components include date wheels, date change levers, a day wheel and day-of-the-week lever, as well as an intermediate month wheel with month rack disk and finger. Some watches also offer year and decade disks, as well as a moon phase indicator. The entire labyrinth of disks and levers is linked, so that every night a switching impulse is transmitted from the main movement via a tiny lever, pulling on the date change lever. The impulse causes the date wheel to advance by one position. Simultaneously, the day-of-the-week lever pushes the day wheel, and so on.

Certain difficulties arise in making perpetual calendar watches with regard to setting the functions and readouts. Thus, most brands’ perpetual calendar watches differ slightly in the configuration of their movement parts compared to those of other brands. Many of today’s finest watchmakers have patents on their mechanical systems for setting the dates and times, for unusual readouts, extended time indication, or added functions such as chronographs, astronomical indicators, or multiple time-zone indicators.

Ulysse Nardin, for instance, holds several patents on its El Toro +/- GMT Perpetual Calendar, which combines the GMT (Greenwich Mean Time) function with the perpetual calendar and adjusts both forward and backward in seconds via the quick-setting corrector on a single crown. The hour hand adjusts instantly with the use of the + and – pushers when travelers move between time zones. Similarly, A. Lange & Söhne’s Datograph Perpetual watch, which consists of 556 parts (including a 48-step perpetual calendar cam), combines a column-wheel chronograph and a fly-back jumping 30-minute counter function. This watch is accurately computed to a rate deviation of only one day in 122.6 years.

“In the design of perpetual calendar watches, there are three major challenges for which we have developed responses,” says Wilhelm Schmid, CEO of Lange Uhren GmbH, makers of the A. Lange & Söhne brand. “For superior legibility, we have combined the Lange outsize date with the perpetual calendar. To make it as user-friendly as possible, the mechanism needs only to be advanced by one day in 2100 at the push of a button. To make sure that the switching of the calendar indications does not affect the rate of accuracy of the movement, we have devised a way to build up the energy for this process over a period of 24 hours.”

One of the more beautiful readouts is the moon phase indication, which operates via a small disk integrated into the complex movement. As the disk rotates it reveals, through an aperture on the dial, the moon phase for the month in progress. A handful of brands have developed new moon phase indicators that can depict the moon in both hemispheres. IWC Schaffhausen’s Big Pilot’s Watch Perpetual Calendar Top Gun offers a stunning rendition of two moon phases (one for each hemisphere) in a circular disk at 12:00 that offers dual drama.

Calendar systems in timepieces date back to the early 15th century, when astronomical clock towers inspired clock makers to incorporate additional time-tracking functions into their smaller clocks. Watchmakers of the 18th and 19th centuries wrestled with difficulties inherent in calendar watches, such as how to automatically advance the date or how to accommodate leap years. While Breguet registered a perpetual (self-winding) watch with calendar in the year 1795, the development of the perpetual calendar system is generally credited to Jules Louis Audemars in 1853 (though the original invention was not available and operational in a watch until 1860). The initial system, since perfected by several companies, features a circular cam consisting of 48 months accounting for differences in dates. Prior to the advent of the perpetual calendar, most calendar clocks and pocket watches required some sort of manual adjustment at the end of each month to accommodate the change between 30-, 31-, 28-, and 29-day months. As mechanics were miniaturized and pocket watches morphed into wristwatches, perpetual calendar wristwatches progressed, yielding today’s high-technology pieces.

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