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What are calendar watches?

The lengths of the months may be irregular but mechanical watchmaking has already solved this issue with calendar complications of varying accuracies.

What are calendar watches?

The archetypal perpetual calendar display format. (Photo: Vacheron Constantin)

SINGAPORE: Counting the days with a calendar isn’t easy. Although our calendar marks complete days, the actual length of time the earth takes to revolve around the sun isn’t some convenient whole number, whether measured in hours or days. This difference accumulates over time and causes a divergence that necessitates occasional correction. As if that’s not enough, the modern Gregorian calendar that we use is the result of numerous revisions over the course of history. Case in point: August has 31 days simply because Julius Caesar wanted it to, as several of his achievements took place in that month.

The quirks of the Gregorian calendar begin with the uneven lengths of months – seven of them have 31 days, four have 30 days, and February alone has just 28 days. Leap years come next. On leap years, February has an extra 29th day, called the leap day. Contrary to popular belief, leap years don’t occur regularly either. Although a leap year generally occurs every four years, a year that is divisible by 100 is not a leap year, unless it’s also divisible by 400. So, neither 1800 nor 1900 was a leap year, although 2000 was; looking ahead, 2100 will not be a leap year.

Keeping track of the day and date is critical to society’s functioning, so one will expect watches to be capable of doing so besides telling the time. Indeed, the calendar complication is ubiquitous today, and modern timepieces are more likely than not to come with them, simply for convenience’s sake. Not all calendar complications are created equal though – the more accurate ones will obviously require fewer corrections over a year, but end up being more mechanically complex and costly.


A window aside, a hand can also be used to indicate the date, like this Montblanc Heritage Chronométrie Collection Twincounter Date. (Photo: Montblanc)

The simple calendar is the most basic form of the calendar complication, and is found in almost all quartz and most mechanical watches today. Its mechanism is straightforward – a wheel draws energy from the watch’s main gear train, and releases this energy to advance a disc (or hand) once a day at midnight. Because the simple calendar cannot account for the differing lengths of the months, it must be manually corrected by the wearer at the end of every “short” month with less than 31 days. Advancing the display to the 1st of the following month is thus done five times a year.


The IWC Portugieser Annual Calendar displays the day, date and month in three apertures arranged in an arc at 12 o’clock. (Photo: IWC)

The annual calendar improves on the simple calendar by requiring an adjustment just once every year, which also explains its name. At the heart of this improved mechanism is a cam with 12 notches of differing depths that correspond to the months of the year. Essentially, this cam stores the mechanical “programme” that a component called the finger “reads” to advance the dates correctly. Regular months with 31 days have shallower notches, so the date is advanced just once, from the 31st to the 1st. The short months with 30 days, however, have deeper notches that cause the finger to advance the date twice, from the 30th to the 1st, thus requiring no adjustments by the wearer. As the annual calendar cannot account for February’s extra short (and variable) length, the complication must be manually corrected every February.


The Audemars Piguet Royal Oak Perpetual Calendar has the full suite of calendar information on display, with the addition of a moon phase indicator to top things off (Photo: Audemars Piguet)

The perpetual calendar sits at the top of the pecking order, by being able to account for both the differing lengths of the months, as well as leap years and their effect on February’s length. It is, however, exponentially more complex compared to the annual calendar, as an entire system of cams, wheels, pawls and levers is involved. Because of this, the perpetual calendar is considered a high complication – a complication that’s significantly more difficult to execute given its mechanical complexity. The end result however, is quite a marvel to behold: Assuming that the watch is kept running, the date will only require a manual correction in 2100, when the mechanism is unable to account for the lack of a leap day in February. Considering the average human lifespan, this is practically forever, hence its name. (In practical terms, however, mechanical watches require servicing every five to 10 years, so the calendar displays will be reset by the watchmaker anyway.)


Although it also displays the day and month, the Vacheron Constantin Historiques Triple Calendrier 1942 is a (simple) triple calendar, not an annual or perpetual calendar. (Photo: Vacheron Constantin)

It is important to note that the astronomical information displayed on a watch does not necessarily correspond to its calendar’s complexity. Although the annual calendar tends to indicate the day and month, and the perpetual calendar tends to also have the year (and leap year) on display, there are watches that buck this trend. The triple/complete calendar, for instance, displays the day, date, and month, but is at its heart is a simple calendar that still requires manual adjustments at the end of every short month.

Source: CNA/bt