Technical: Perpetual Calendars

Ask a panel of watch enthusiasts what the most useful complication on a watch is, and chances are they'll tell you it’s the date function. The self-changing date was invented by Rolex in 1945, so perhaps it might surprise you to discover that the perpetual calendar, one of the most impressive complications ever designed, made its first appearance on a wristwatch twenty years before that. Strangely, the movement in that wristwatch came into existence in 1898, over a quarter-century prior, making one of the most important movements of our time the one that almost got away...

The movement in question was Patek Philippe's 97975, which spent the first twenty-seven years of its life nestled inside ladies' pendant watches, before being transplanted into a wristwatch in 1925 at the request of watch connoisseur Thomas Emery. He recognised the greatness of this little movement, and today the perpetual calendar is Patek Philippe's signature complication.

But what is it that makes a perpetual calendar so impressive? To understand that, we need a quick history lesson. Early Roman calendars were based on the lunar cycle, and had ten months. These months all had thirty days, except the last month to be added - February - which was docked two days to synchronise with the moon. Then, during the reign of Caesar Augustus, the solar calendar was introduced and two more months were added. Some of the months gained a day (the ones named after Roman Emperors) and every fourth year became a leap year to match the solar cycle. To make things even more confusing, at the turn of every century a leap year was skipped to stop the 'leaps' veering too far the other way. Pope Gregory XIII, however, decided to tweak the calendar again, because there was still a fractionate amount of slip. The concern was that Easter was moving away from the spring equinox, so by adding a leap year back to every 400th year, the accuracy of the calendar was improved further.

His Gregorian calendar is the bane of a watchmaker's existence, and despite over a century of development, even the most accurate perpetual calendar watches still need adjustment when centuries turn without a leap year. Despite this, a perpetual calendar can differentiate between leap and non-leap years the rest of the time, allowing it to display the date, day, month and year correctly. So how does it do this?

The heart of the perpetual calendar is the program drive wheel (or cam). Most perpetual calendars work in a similar way to IWC's impressive crown-operated calibre 51614: its program wheel is a gear with four years' worth of square teeth, with each tooth representing a month. The teeth vary in length for the duration of the corresponding month, with the February on the fourth year being slightly shorter than the others to accommodate the extra day. The lever running to the program wheel 'feels' the depth of each tooth and skips the days forward at the end of the month accordingly.

With thousands of parts working in unison, it's obvious why the perpetual calendar is so highly revered. With the next adjustment not needed for the best part of a century, lucky owners (and the generation that follows them) can enjoy over 20 billion beats of their perpetual calendar's heart before they even need to lift a finger.