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118. That’s how many components had to be added to create the ‘in-line’ day/date/month display mechanism of Patek Philippe’s latest perpetual calendar, reference 5236P. The mechanism incorporates an ultra-sophisticated system for which three patents have been filed.

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Calendar-related displays were a very early addition to pocket watches—long before minutes and seconds hands began to appear. In the late 16th century, astronomical watches featured indications such as the date, day, and month (and its length). But these were still ‘simple’ calendars that required manual correction every time a month had less than 31 days.

A mechanical memory of 1,461 days

It was only two centuries later that the first calendars known as ‘perpetual’ appeared. In order to display the correct date while automatically taking the different lengths of the months (28, 30, or 31 days) and the succession of leap years into account, a perpetual calendar must be ‘programed’ to follow a cycle of 1,461 days (365 + 365 + 365 + 366). To coordinate these different displays, watchmakers developed a very sophisticated cam-and-lever mechanism that pivots on its axis.

Any self-respecting perpetual calendar has at least three displays: day of the week, date, and month. These are generally joined by a leap-year cycle indicator (1, 2, 3, 4) and a day/night indicator—two very useful functions for setting the calendar. There are essentially two types of day, date, and month display and they can sometimes both be found on the same watch: hand-based displays or disks in apertures. Recent decades have also seen an increase in alternative display methods (such as rollers).

A first for a Patek Philippe wristwatch

As the ultimate classic grand complication, the perpetual calendar occupies an important place in Patek Philippe’s collections. In its current ranges, it assumes a variety of different guises depending on the movement: hands-only display with three auxiliary dials (caliber 240 Q); a blend of a dual day-month aperture at 12 o’clock and a hand-based date display at 6 o’clock (caliber 324 S Q); and a combination of a day aperture at 9 o’clock, a month aperture at 3 o’clock, and a central hand ‘retrograde’ date display, meaning that it returns to its starting position after tracing an arc on the dial (caliber 324 S QR).

On perpetual calendar reference 5236, Patek Philippe wanted to innovate by using the display method that is the most straightforward for users and the quickest for them to read: day, date, and month grouped together in a single ‘panoramic’ aperture at 12 o’clock. Given its difficulty, this ‘in-line’ display had never previously been offered by the Geneva-based manufacture within the limited space of a wristwatch. It had, however, been presented on a pocket watch from 1972 intended for the American market—with one distinctive feature: the indications were arranged in the month-date-day order used in the United States. But to miniaturize this system to fit into a wristwatch, Patek Philippe had to develop a new display mechanism, which alone involved 118 components and three patent applications.

Four disks in a single line

The first challenge was readability. How could the indications be made large enough to read easily while taking up as little space as possible to retain an elegant case (thick wristwatches not being Patek Philippe’s cup of tea)? The display of the ‘American’ pocket watch consisted of three disks: one for the month, the date, and the day. If miniaturized and transposed to a wristwatch, this system would have yielded a date that was too small.

The designers therefore decided to use two disks for the date, one for the tens and another for the units. The difficulty then was to make sure that the four disks could turn independently of one another while appearing on exactly the same horizontal plane, without overlapping or touching each other. To achieve this, Patek Philippe developed a system with two ‘coplanar double ball bearings’ (first patent application). In other words, the display mechanism consists of two assemblies, each featuring two disks on ball bearings, one slotted concentrically inside the other. On the middle left is the day disk, with its seven abbreviations (MON, TUE, etc.), surrounded by the date tens disk, which repeats the sequence 0-1-2-3 five times. On the middle right is the month disk, with its 12 abbreviations (JAN, FEB, etc.), surrounded by the date units disk, which repeats the numbers from 0 to 9 twice.

The second challenge was safety. How could the designers make sure that the two date disks remain perfectly synchronized and aligned, and prevent a correction or a jolt from making a disk jump twice instead of once? For the tens, Patek Philippe incorporated a ‘Maltese cross’ component, already common in watchmaking, with a special shape that prevents any subsequent movement once a command has been made. However, for the units disk, the designers developed an innovative ‘self-locking’ system (second patent application). This mechanism consists of two overlapping wheels, one of which is ‘elastic’ and fitted with a ‘jumper’ (spring). When the movement causes the units to change, both wheels turn together. But as soon as it makes this jump, the ‘elastic’ wheel shifts slightly with the action of the spring. As a result, the other gear components come into contact with the elastic wheel, which prevents any further advances until the following date jump.

The ingenious transition from 31 to 1

The third challenge (and third patent application) was the transition from the 31st to the 1st of the following month. The unit 1 normally jumps to unit 2 (01>02, 11>12, 21>22). But when 31 changes to 1, the unit 1 must not move. So how can it be made to stay in place for two consecutive days? Here too, Patek Philippe developed an original system, this time using the shape of the wheels and the gear ratios. The engineers thus removed teeth from some wheels and, in one case, introduced an oversized tooth to ensure optimal operation.

Full of energy

Another difficulty was to ensure an ample energy supply—forever a concern in watchmaking. To limit the energy consumption of the in-line display module, most of the geartrains were ‘jeweled’, meaning that their spindles pivot between synthetic ruby bearings (the famous red jewels often seen on movements), which reduces friction.

There is still so much to say about the new self-winding movement that incorporates this display. Particularly in terms of the way in which Patek Philippe has ‘boosted’ it to drive its nine indications (including seven disks): central hours and minutes; subsidiary seconds at 6 o’clock; in-line day, date, and month at 12 o’clock; moon phases at 6 o’clock; day/night indication in a small round aperture at 8 o’clock; leap-year cycle in a small round aperture at 4 o’clock. But by looking at the in-line display module alone, it goes to show just how much simple and easy-to-read indications involve great design challenges behind the scenes in fine watchmaking.

Did you know?

Perpetual calendars aren’t, in actual fact, entirely ‘perpetual’. In the Gregorian calendar, if the first two digits of the first year of each century (2000, 2100, etc.) can’t be divided by 4, it is an exception to the rule and isn’t a leap year. This will be the case in 2100, 2200 and 2300, when perpetual calendars will have to be manually advanced by one day.



A specialist in watchmaking journalism, Michel is the founder of The Next Hour. Fascinated by the fundamental issues related to communication and social networks, convinced that watchmaking still has a very large audience to capture beyond the circle of connoisseurs, he launched The Next Hour to chart a course and make this medium a fun and informative meeting place of quality. Michel is also a member of the Cultural Council of the Fondation de la Haute horlogerie and a member of the Grand Prix d’Horlogerie de Genève Academy.