Horological Meandering

Calendar watches: Differences between simple, full, annual and perpetual calendars

 

 


On numerous occasions, I am asked about the different calendar mechanisms that are offered in various mechanical watches, and often there are misunderstandings, regarding their function and display. First all, it is important to note that we have to differ between the calendar mechanism located under the dial, and the calendar display on the dial itself. Not necessarily, the dial display reflects the mechanism's true complexity, and vice versa. The dial display can be simple, yet it is driven by a perpetual calendar mechanism, while other dials appear wonderfully complex, making the spectator think that there must be a very fine and complicated mechanism behind it, while in fact it is all but a show, made to impress people and stimulate the purchase decision. The latter is often the case with cheaply made watches, mostly of Chinese origin:


This watch desperately tries to appear like something it isn't: a perpetual calendar watch. It looks busy, very complicated, with lots of calendar indications. In fact, its Chinese movement has but a very simply day-date mechanism. The rest is mockery:  What appears to be a moon phase display, is a simple day-night indicator, coupled with the 24 hours hand. The normal hour wheel drives another wheel with twice as many teeth, so it rotates once per day. That's all. The windows showing the month, and even the year, have no functionality at all. Their displays have to be advanced manually, by means of large pushers. These oversized corrector pushers also have the wanted effect to make unsuspecting spectators believe this watch is a chronograph. While the brand depicted above is a legitimate business, this pusher design is also very popular with counterfeiters of well-known expensive chronographs: instead of real start-stop and reset mechanisms, the pushers only advance date and weekday displays.

On the other hand, this absolutely simple-looking watch houses a real perpetual calendar mechanism:



As a consequence, we have to avoid the conclusion that "what appears complicated, must be complicated", and concentrate on the calendar mechanisms hidden under the dial. All watch calendar systems are based on the rotation of the hour wheel, that normally needs 12 hours to complete, as it drives the watch's hour hand. Since the calendar changes but once per day, this rotation has to be transformed into one every 24 hours, by adding an intermediary wheel with the double number of teeth as the hour wheel. This 24 hour wheel can be used for several purposes: for a 24 hours hand, a day/night indicator, or you equip this wheel with a finger, that serves to advance a date ring once a day. That would be the


1. "Normal" or monthly calendar: has a date display (either window or hand) from 1 to 31. Changes the date every midnight, but has to be corrected manually in every month with less than 31 days.



1.a. "Day-date": Adds a weekday display to the date, but the mechanism is still the same: at the end of every month with less than 31 days, it has to be corrected.

This weekday indication is also advanced by the 24 hours wheel, that simply received another finger, switching the date and weekday indicators at (roughly) the same time.



1.b. Moon phase: The 24 hours wheel can also be used to advance a moon phase display. Here, the problem lies in the fact that a complete cycle of the moon can not be divided into full 24 hours periods, Consequently, one daily step of the moonphase display will sooner or later result in a divergence between the watch's indication and the real phase of the moon. Please learn more about the moon phase accuracy problem and the watch industry's efforts to solve it by clicking here.

The moon phase display became a popular decorative element on watches, so it is not unusual to find such indications even without any other calendar displays. But much more often, it is combined with a small pointer calendar:


1.c. "Full calendar": Adds a month display, and - sometimes - also a moon phase.

Based on the 24 hours wheel and the date ring, other calendar indications can be realised. A so-called "full" or "complete" calendar features the indications of at least the date, weekday and month, sometimes combined with a moon phase.



The mechanism's intelligence is still the same as on the simple calendar watch, but additionally, you have to adjust the correct month and the moon phase. Some movements do switch to the next month whenever the date jumps from 31 to 1 of the next month, but there are also movements where the month display does not have any automatic gear at all, but has to be advanced manually every month.


2. "Annual" calendar: This mechanism derives its name from the fact that it has be manually adjusted by the wearer only once a year. Contrary to the standard watches with date display, these calendars already need some kind of mechanical program: the sequence of months with 31 and 30 (special case February is now treated as 30) days is as following: X - X - X - X X - X - X. The gearing is pre-programmed to show all 31 date numbers in months with X, but to quickly pass over the 31 and immediately switch to 1 from 30 in months with -. Unfortunately, the February is a special case, normally having 28 days, with a 29rd day every four years (leap year). This month is not recognized in the program, so every year, the owner has to manually interfere, by changing from 28th (or 29th) of February to the 1st of March.

When Patek Philippe presented its Ref. 5035 annual calendar in 1997, it stirred quite some attention, since this complication was barely known before:



These annual calendars are still rare, but a highly interesting complication. For correct use, they only need a month display aside the date:


Sometimes, other indications, such as weekday and/or moon phase can be added, but do not affect the classification as "annual calendar".


3. "Fouryears calendar": These are really rare, I am aware of only an older Breitling offering this complication. The "program" of the annual calendar is extended her, by recognizing the February as a month with 28 days. As a consequence, the owner needs not to interfere with the mechanism for four years. Only in leap years, he has to manually change the February from 28 to 29.

4. Perpetual calendars: The next step is the extension from recognizing the 28 days February every year to the recognition of the one February with 29 every four years (leap year). Counting the program interval, this means that the watch has a full four years program, that is constantly repeated. Thus, it is not really a "perpetual" calendar, because our Gregorian calendar system will drop a leap year every 100 years. Consequently, nearly all mechanical perpetuals on the market will have to be manually corrected every 100 years.

It is some kind of generally accepted dogma that perpetual calendars are complicated watches and have to look complicated:


Another possibility is to indicate the civil calendar year itself:


Again, the designation as a "perpetual" is based solely on its mechanical program, and not on the number of indications.The number and nature of dial indications is not necessarily connected with the fact that a certain watch is a perpetual calendar. This Moser perpetual, for example, only shows the date and the month, neither weekday, nor moon phase is shown:


5. Secular calendars take this over the top, by even knowing this 100 years rule, and are independent from user interference for 400 years even, when the calendar arithmetics enforces an exception from the 100 years rule (happened in 2000, luckily for most owners of perpetual calendars). The only wristwatch featuring a secular calendar, at least the only one I know about, is made by Svend Andersen:



6. Astronomical calendars: A very special breed of calendar watches are those that connect the passage of the days and months to the astronomical movement of Earth, moon, the planets and/or the stars. Here, the main problem is that the movements of the celestial bodies do not observe our civil calendar's strict division into 24 hours intervals. For an astronomical watch, a year does not pass in exactly 365 days, with an additional day every four years, but in roughly 365.25 days, needing about a quarter of a day longer than a standard calendar watch. Therefore, the display of a date is either completely omitted, or needs a highly complex mechanism, to "translate" the astronomically correct display into the incorrect - but common - date indication.



The useability issue: Setting a perpetual calendar watch

Unfortunately, the theoretical accuracy of the calendar mechanism has not much to do with the physical reality: Whenever a watch is stopped for a time, all indications have to be newly set. It is then a question of the individual mechanism's ergonomics and usability whether this procedure is simple or complicated, or can even damage the delicate mechanism when something is done wrongly.

The majority of perpetual calendars, regardless of the number of their indications, offer a correction pusher for every indication. Thus, the owner can "compose" every possible date, including the moon phase. This enables him to keep his watch in hand, even in those infamous "exception years" mentioned above, making the necessary adjustments himself. The problem is that in most cases, you simply cannot stampede towards the pushers however you like, pressing them in arbitrary sequence; one has to follow a very precise procedure, and otherwise risks a costly repair.

Therefore, some manufacturers produced a more "foolproof" system, by firmly coupling all indications together. This means that the watch knows which weekday is the 5th of March 2012, and which phase of the moon this day has. All the user has to do, when the watch has stopped for a while, is adjusting the indications forward until the correct date is shown. This system's disadvantage lies in its rigidness: If the owner, full of enthusiasm and energy, "overshoots" the current date, his only option is to leave his watch inactive, until the desired day has come (I call this the "Sleeping Beauty-problem"), or to send the watch to the manufacturer, asking to adjust it to the correct date, which is of course a costly solution. Also, these watches have to be sent for adjustment every 100 years to correct the mechanism in the exception years.

IWC's Da Vinci is a prominent example of this system:


A few manufacturers opted for a system that permits a simple adjustment by the owner, by decoupling only one indication, mostly that of the weekday. Setting the weekday independently is somehow a compromise between the "adjust every single indication" approach and the foolproof full program. This system permits a flexible and relatively simple adjustment by the user, while keeping the mechanism user-friendly and solid.





 

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