The 31st Of The Month: How Perpetual Calendars Cope With Crowded Numbers
The latest caliber 7138 perpetual calendars from Audemars Piguet take a novel approach to handling cramped numerals, but the basic problem is as old as perpetual calendars themselves.
One of the oldest and most intractable problems in perpetual calendar design has to do with the numerals for the date. In a traditional perpetual calendar, the date is usually indicated by a hand in a subdial (often with a moonphase disk) which points as the days pass to the numbers one through 31 (skipping over 29, 30, and 31 in February, and 30 and 31 during February when it’s a leap year). The basic problem this presents compositionally, is that the hand pointing to the date passes through exactly the same radial distance every time it jumps. That means that once you get into the double digits, you have to figure out how to fit two digits into the space normally taken up by one, and once you get into the twenties and thirties, you don’t just have to fit in a nicely svelte one – you have to jam in a curvaceous two or a callipygous three. This in turn means that as you go around the date subdial on a conventional perpetual calendar, the numbers look more and more like commuters on a train at rush hour. Moreover, with the 31 and the 1 so close together, it is hard to not see an indication for the 311th of the month. What is an enterprising watch designer to do?
The first thing you can do is simply not worry about it at all. You can claw back a little extra space by making the numbers smaller, although this starts to impact legibility which, if you are showing the date in a subdial, is probably challenging to begin with but perhaps we can assume that if you can afford, say, a vintage Patek Philippe ref. 1526, that you have people who keep you apprised of the time in any event and that it is the historical, technical, and general aesthetic importance of the timepiece with which you are concerned rather than legibility per se. There is a certain appeal to the time-honored, aristocratic disdain for the problem and indeed the refusal to see it as one, but horology is nothing if not the art of incremental improvement on essentially mature technology, and so watchmakers and dial designers have experimented with alternatives.
The obvious alternative is simply to give yourself more real estate to work with. Pocket watches for instance, give you enough room to spread out that the whole question of crowded numbers almost vanishes. It doesn’t completely as the issue of the date hand moving through the same arc for every date remains, but certainly the problem is diminished and legibility is improved.
The other obvious alternative is to use a different date display. Crowded numerals become basically a non-issue if you use a retrograde date display, and the problem such as it is, vanishes completely with digital date displays (the Lange 1 Perpetual is a case in point). However, jumping digital perpetual calendar displays pose their own unique challenges – including the problem of managing power distribution to the date disks when both of them switch over at the end of the month, and adapting a perpetual calendar mechanism to indexing two date wheels rather than a single date hand. And then there’s the undeniable appeal of a traditional perpetual calendar display, as we came to know the complication in the Patek 1526.
One simple and often encountered solution is to color the 31st of the month in red. Patek historically has tended to avoid doing this although other perpetual calendar makers have done so in the past. Audemars Piguet has used a red number 31 on the Royal Oak Perpetual Calendars since 2015, and continues to do so in some current models, although it has explored other design options as well.
One of these is the use of a staggered 31, with a 3 and 1 offset from each other, in the John Mayer AP Perpetual Calendar.
The use of red for the 31st has some very deep history with AP; it’s the choice that Jules Louis Audemars made for the 31 in his perpetual calendar school watch, completed before the founding of Audemars Piguet.
In its newest perpetual calendars, based on the caliber 7138 (which allows all indications for the calendar to be set individually from the crown) AP’s gone for another solution to fitting in the numbers so that they’re evenly spaced, and so that the 31st and the first are easy to tel apart. The usual arrangement in perpetuals with subdials for the date, is to begin with the numeral 1, and then follow it with a dot and as you go around the subdial, with alternating odd numerals and dots for even numbered dates. This convention is what produces the somewhat awkward juxtaposition of the 31 and the 1. In the new perpetuals, AP has chosen to start with the odd numerals, 1 through 7, and then switch over to even numbers starting with 8.
This means that the last numeral show is 30, with the 31st represented by a dot and the upshot of the whole design is that there’s no awkward juxtaposition of the 31 with the 1, and the overall spacing of numerals around the dial seems more even as well. The compromise of course is the placement of the numeral 7 and the numeral 8 right next to each other, but Audemars Piguet obviously felt that the trade-off was worth it to get a more even distribution of numerals around the dial, and also to avoid having the 31 and the 1 butting right up against each other. (Audemars Piguet felt the problem was important enough to also redesign the date wheel; the teeth of the caliber 7138 are unevenly spaced in order to compensate for the increasing width of the numerals as the month goes by).
There is probably no perfect solution to balancing the spacing of numbers in a traditional perpetual calendar subdial and I suspect the problem, to the extent that it is a problem, will remain an open and interesting one for watchmakers to tinker with in the future. Alternatives to the traditional 31 day subdial are many but figuring out how to approach the challenge is something any maker of perpetual calendars in the classic format will need to consider. Of course, there is an argument to be made that the simplest solution, which has been around in wristwatches and pocket watches for a century and half, is to just let the 31 and the 1 sit cozily next to each other, with one or the other perhaps picked out in a different color. Still, the ingenuity expended in addressing the problem is I think part and parcel of the perpetual calendar as a complication – in practical terms, a practical benefit over a simple calendar only incrementally, but still a demonstration of the pursuit of every possible improvement, no matter how small, which is intrinsic to watchmaking.