Calendars in Watchmaking - The History of Perpetual Calendars


Calendars in Watchmaking - The History of Perpetual Calendars

We all know that our year is made of 365 days, and has months that have a different number of days - and one of them, February, also changes every four years. As you can readily understand, this was a real issue for the watchmakers wanting to record and show not only the time on the clocks and watches but also the date.

So, where did this issue come from?

This division started with the Romans. In the ancient times, they had a year divided into ten different months, plus some intercalary periods, Curiously, the length of the year was decided by the Pontifex (a member of a council of priests in ancient Rome), so this system eventually originated confusion.

Roman Calendar

Under Julius Caesar, who was Pontifex Maximus in 46 BC, the calendar was reformed so to be more accurate. The new system, called Julian calendar, was a more precise solar calendar with 12 months. 

Apart from the traditional ten months, with a February of 28 days, the two new ones were named to honor the Caesar dynasty. This is why the two new months assumed the names of July and August,  after Julius and Augustus.

These two new months rendered the other four months inaccurate as for the meaning of their name. September, which was the seventh month, as its name derived from “Septem,” became the ninth, and so on. 

This new system was much more accurate than before. However, it did not take into account that solar time is six hours longer than what was provided by the “basic” Julian calendar.

This problem was resolved only 1,500 years later. 

The calendar we use today has been introduced in 1582 by Pope Gregory XIII and was an update of the Julian Calendar, it is named “Gregorian” after him.

Pope Gregory XIII

This system was introduced after the calculations made by astronomers determined that the Julian calendar was imprecise - so much that in 1,500 years of its use, there was a gap of several days against the astronomical time.

Gregory XIII was not interested in astronomy per se: he just wanted that Easter fell as close as possible to the spring equinox every year. His solution was to add an extra day every four years to keep the calendar synchronized to the seasons.

But... there is always a but. And we will discover it a bit later.


The first perpetual calendars


If you think that this kind of measuring is something that came later in horology, think twice. The first perpetual calendar was made by none other than Thomas Mudge.

Thomas Mudge

Thomas Mudge, who was born in 1715 and died in November of 1794, was an English watchmaker who studied under George Graham. He’s the one who invented several critical elements of the modern watch, including the lever escapement in 1755. Among the others, we can remember the mechanisms for the equation of time, minute repeater, and also, the perpetual calendar. 

He became very famous in life, as he was directly commissioned to supply watches for Ferdinand VI of Spain, producing at least five watches, including one that repeated the hours, quarter hours, and minutes.

Mudge created the oldest known perpetual calendar watch in 1762.

Thomas Mudge Perpetual Calendar Pocket Watch

It was, in fact, a pocket watch: this one above, which now resides in the British Museum. 

Thanks to his invention, date keeping became much more widespread, to reach the modern days where the simple date and day features are decidedly commonplace.


The modern evolution of date keeping


The date keeping mechanisms in watches are of different kinds, from the simplest to the most complex - and of course, the price of said watches depends on the complexity of the mechanism.

The regular watches with a simple calendar function are almost straightforward to make. The day/date mechanism is so well-tested and commonplace that it almost does not add much to the cost of a watch. Even if the simplest models mount a double date function, showing both the number of the day and the day name, the date complication is well-appreciated in horology, and has been instrumental in the design of many luxurious references.

For example, check this beautiful Glashutte Original Senator Panorama Date, which combines the elegant and unexpected design of its dial, and its double-rotor date display, with a moon phase indicator.

Glashutte Original Senator Panorama Date

The watch is shown in all of its glory here:

The real issue is that these simple mechanisms have no way to track months with 30 or 31 days, so you have to adjust them manually in April, June, September, and November (apart from in February, that is). Most of the simpler watches feature this simple date and day function, or just the date number. 

A step upwards is the so-called annual calendar. 

The annual calendar function has been introduced at the end of the 1900s. It is also called triple date, and can track the months with 30 and 31 days automatically, but cannot add one day to February during the leap day every four years.

One of the watches mounting this complication is the beautiful Corum Admiral’s Cup Legend 42 Annual Calendar.

Corum Admiral’s Cup Legend 42 Annual Calendar

As you can see, it is delightful, sporting the iconic 12-sided design typical of the watchmaker. The date is shown through a sub-dial at six, for the months, and a central date hand indicating the day number over the elegant dial with a vertical barleycorn guilloche motif.

You may find it here

So, if you own one of this kind, you will have to adjust it manually just once per year, in February.

The third - and best - group of watches is what mounts the so-called perpetual calendar. That is, through an intricate series of mechanisms called equation of time, they are capable of tracking also the leap year procession. 

Some are so advanced that they can calculate the deviation of time, which is not addressed by the Gregorian Calendar - because our current system is almost perfect, but not entirely.

So, a perpetual calendar should be, as you can readily see, err, perpetual. It has ways to address this problem, so the calendar year stays consistent with the astronomical year, and they will work until the year 2100.

Audemars Piguet Classique Perpetual Calendar

What you see here is an example of a watch with a perpetual calendar: the Audemars Piguet Classique Perpetual Calendar, a gorgeous, elegant platinum-cased watch if there’s one, displaying its complex and mesmerizing caliber through the glass display back.

You may find it at this link

There is another level of madness in date-keeping precision: the ultimate kind of watches mounts a correcting mechanism that addresses the BIG issue of the calculation of the centennial leap year. 

And we are talking about the secular perpetual calendar watches. There are only a few in production - they are exceptionally rare - and well, if perpetual calendars are exceptional, these watches reach a level of untouched horological insanity.

So, what is a centennial leap year? The Gregorian calendar, in all of its complexity, is not perfect. There is a little residual error, which is corrected by the Gregorian cycle every hundred years. 

In short, to correct this tiny residual mismatch, the Gregorian system also introduced a unique, secular leap cycle. Only the secular years which are divisible by 400 are considered to be leap years. Therefore, the secular years 1700, 1800, 1900, 2100, 2200, 2300 weren’t or will not be leap years, and have only 28 days in February, while 2000 was, and 2400 will be.

As you can imagine, this additional level of complexity, while addressing a minimal error, wreaks havoc in the ordered mechanical dreams of the typical watchmaker. 

The presence of this further exception means that perpetual calendars need to have an additional regulation that knows precisely when the leap year happens and when it doesn’t with respect to the secular period. 

So, if combining gears and wheels to calculate a “simple” perpetual calendar was not the easiest of tasks, allowing a mechanism to calculate this exception becomes almost hellish. 

But some manufacturers managed to achieve such a feat.

Svend Andersen Perpetual Secular Calendar

The one shown above is one of the very few wristwatches in production (currently, there are three in history) mounting this uber-exotic complication: the Svend Andersen Perpetual Secular Calendar. This watch is programmed to be precise up to the year 2400, a feat which some of you might find superfluous, since to fully exploit from it. the watch would have to work continuously from now to then - and you are well-aware that a watch should be serviced every five to ten years… so even this horological marvel would have to be serviced at least 38 times from now to then!

You can find one of this exceedingly rare birds on sale at, here


The 10,000 Year Clock


If you think that there cannot be anything more precise than this, you are wrong. Even if we are not talking anymore about a wristwatch here. 

There is a project to build a (massive) mechanical clock that is going to work for 10,000 years.

This ambitious project is supported by none other than Jeff Bezos, the richest man on Earth.

The 10,000 Year Clock is being created by the Long Now Foundation. This clock aims to work for 10,000 years - and since the first traces of our modern civilization date back from around just 6,000 years ago or so, you can understand this is an ambitious undertaking.  

10,000 Year Clock

The clock is designed to generate power for itself through solar power and thermal energy, it is believed these sources alone will generate enough power to tell the time, but visitors will be encouraged to wind the clock to display the current time. This process will both save energy and give the possibility of checking when the previous visitors were there.

As of today, the clock is still on its building stage - but prototypes like the one shown above have been built to show how the final assembly will be.

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