Wheels within wheels

The job of the perpetual calendar sounds pedestrian – to show the right date, every day – yet wristwatches with this capability are among the most coveted by connoisseurs and collectors, with auction prices for rare models by companies at the top of watchmaking’s pecking order running into millions of dollars. So why do perpetual calendars inspire such fanatical fascination?

Their origins lie in the problem of measuring time. A year is 365.2425 days long, and over time those fractional days add up. In 1582, Pope Gregory XIII dealt with the problem by decreeing that the known world would skip ten days, add an extra day to February every fourth year – a leap year – and skip the leap year once every hundred years. His scheme aroused howls of protests from tenant farmers, who thought landlords were short-changing them, and suspicion among Protestant princes, who suspected it was a nefarious plot to subject them once more to the papal thumb, but today the Gregorian calendar is more or less the world standard.

If you’ve owned a wristwatch you’ve probably noticed that most of them have a date mechanism with numbers going from one to 31. This means you have to correct the date by hand at the end of every month that isn’t 31 days long; in February, for three out of every four years you have to change the date on the 28th, and in a leap year, on the 29th. This seems like a minor inconvenience at most, but watchmakers being perfectionists, a calendar mechanism was invented that would automatically show the right date, all the time, with no need for manual intervention by the owner.

The first step in building a perpetual calendar is to encode the length of each month in what watchmakers call a programme wheel – a metal disk with 12 steps of varying depths along its circumference. The depth of each step corresponds to the length of each month, and a system of levers and springs “senses” the depth of each step, changing the date from the 30th, 31st, or 28th day of the month (in February) as needed. The leap year is taken care of with a cross-shaped wheel that makes a full rotation once every four years, and which triggers a date change only in a leap year, on the 29th rather than the 28th of February. Integrating all these systems mechanically is immensely tricky.

Thomas Mudge, an English watchmaker, made the first perpetual calendar in a pocket watch in the 1760s; the first in a wristwatch was made by Patek Philippe, in 1925. This was a one-off – basically, Patek took a ladies’ perpetual calendar pocket-watch movement and put it in a wristwatch case. Breguet made a one-off in 1929, which was the first perpetual calendar wristwatch designed from scratch to be a wristwatch, and in 1941, Patek started making perpetual-calendar watches in series – the very first. One of Patek’s first perpetual calendars sold for $2,773,721 in 2009, and it would definitely fetch more today.

In part, it is the difficulty of performing this apparently small task that appeals to collectors. For most of their history, only the top watchmaking companies – like Breguet, Patek and a handful of others – could do it, so perpetual calendars and ultra-high-end watchmaking became synonymous. But it’s also that the perpetual calendar embodies a heavenly order – a little reminder on your wrist of a vision of a beautiful, orderly, clockwork universe.

CGI ILLUSTRATION JOHN HAREWOOD