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Watches and the Pedestrian: Perpetual Calendars

In our new weekly column on watches, we break down the complications of watchmaking.

“Must have been a long winter in Switzerland when they invented that,” someone to my right remarked.

Sure, he wasn’t serious. Still, you have to wonder.

We were in a lecture about perpetual calendars, called Patek Philippe’s Knowledge Series, where they taught people what’s what when it comes to watch complications and history. With all the new perpetual calendars straight out of SIHH 2019, that remark might not have been entirely inaccurate.

So, what exactly is a perpetual calendar?

The short answer is that it’s a calendar complication. It shows you the day, the date, the month, and the year. It automatically adjusts to the correct display, even in a leap year. That means it takes into account all the varying numbers of days in each month (28,30,31), including that extra 29th day in February that just happens to pop up every four years (you have Pope Gregory XIII and his calendar to thank for that).

It’s a simple enough concept to grasp. Your phone does the same thing. But consider this: mechanical watches are mostly just made up of bits of metal, when you strip it down to the basics. There’s no microcomputer to calculate the date, no wifi to send information to it, no memory card to store data.

What that means is that most of this information is “coded” into and processed by tiny gears and cams, each one meticulously shaped to move the hands and numbers into place at the precise time they’re required to. And all of this happens in a very small case that goes on your wrist.

Perpetual calendars differ from the other kinds of calendars that you can find on some watches. A regular calendar displays the date, and needs to be manually adjusted at regular intervals, since it runs on a linear 31-day cycle, making no distinctions for the differing number of days in a month. An annual calendar, on the other hand, automatically displays the date correctly, except during the end of February, when the watch’s date display will need to be adjusted manually.

Most perpetual calendars will not require any manual adjustment of the date until 2100. Why 2100? That’s because every fourth year is only a leap year if it is not divisible by 100. However, if it is divisible by both 100 and 400, then it would still count as a leap year. Again, you can thank Pope Gregory XIII for making our heads spin.

Working this complication into a timepiece is considered one of the highest distinctions in watchmaking. That also means that it is usually found in dress models, though, since perpetual calendars necessarily command a higher price. Last year, though, Patek Philippe came up with the first grand complication Nautilus model, featuring a perpetual calendar. The watch straddles the line between dressy and sporty, and it would likely be just as home in the office as it would be in the outdoors.

In any case, most perpetual calendars will take at least another 81 years before they need to be adjusted (and to see what happens to a perpetual calendar in a year that’s divisible by four and by 100 but not by 400). At that point, you’ll want to take it in for servicing as well, or have your surviving heirs do so, since none of us will be around to find out what happens.

Still, it’s nice to know your watch most likely will.