Watches in Depth - Calendars ¶
Calendar watches are the single most common complication in modern watchmaking. They are so ubiquitous that most people wouldn’t even think they are a complication at all – we’ve become so accustomed to having a date on our watches we take it for granted. The irony in this complacency is that the calendar mechanism can be one of the most difficult and expensive complications to produce when you have to factor in leap years.
The basic mechanical calendar mechanism shows a date through an aperture or extra hand on the dial with no provision for automatically correcting based on the 30-31 day cycle. This is accomplished by having a toothed wheel driven off the base movement that rotates once every 24 hours. The wheel has one extra long tooth that sticks out and mates to a spring-loaded mechanism attached to the date wheel. When the long tooth engages the mechanism, the spring pawl snaps the date forward one notch – the calendar disk (what you see on the dial) having 31 notches. Early date complications had no provision for quick setting – to advance the date (or move it back) you had to roll the hour hands through a full 24 hours for each day.
The next step was to make the date quickset. Today we are accustomed to quicksets by pulling the crown out to the first notch and then rotating. Another older system that is still prevalent is the pushbutton quickset, where a tiny flush-mounted button is integrated into the side of the case. Omega used a different system for many decades where you gently popped the crown out to the second position to instantly advance the date one position (each pull advanced it once, no turning of the crown required). The problem with quicksets that persists to this day is the issue of damaging the mechanism by advancing the quickset during the wrong time. Because the long tooth gear slowly rotates (every 24 hours) the actual action of the date mechanism can take several hours. The danger zone is between 9 and 3 o’clock. If you quickset during that time you risk snapping the tooth off the gear. Nowadays many manufacturers design blocking mechanisms to prevent damage, but many movements still have issues (the ETA/Valjoux series of chronographs like the 7750 are the most commonly afflicted calibres). The solution is to NEVER quickset the date between 9 and 3 o’clock. Always put the hands to 6 o’clock before setting the date.
The next step in the calendar is the day-date and triple date display. A day-date is pretty self-explanatory – the day of the week is displayed along with the date. Essentially the complication of the date is doubled, with two display discs and two advancing mechanisms both engaging at the same time. The triple date takes the day-date and adds a month display that advances every 31 days. It doesn’t factor the 30-31 day cycle, however. The mechanism adds a series of gears that operate a month display that clicks over every 31 days; the principle is the same as a basic date, just with a much longer turnover time.
The next step up in complexity is the annual calendar. Similar to the triple date, the annual calendar factors in the 30-31 day cycle but doesn’t account for leap years. This mechanism has become a signature of Patek Philippe, who produce several annual calendar models. The user only has to correct it once a year, in February. With an annual calendar there is an extra mechanism that will instantly skip over the 31st to the 1st when needed, based on which month is being displayed. An annual calendar is considerably more complex than a simple triple date and commands a significant premium in the luxury market. But it has nothing on the king of calendars – the perpetual calendar.
The perpetual calendar occupies the top tier of complicated watchmaking, being part of the complication trinity that has been a staple of haute-horlogerie for decades – perpetual calendar, chronograph, and minute repeater (nowadays we would add tourbillons to this list as well). A “triple complication” is a watch that combines those three functions - being a chronograph with a perpetual calendar and a minute repeater (see the Patek Philippe 5208 at the bottom of the page for an example). The perpetual calendar is similar to the annual calendar but has the ability to calculate leap years, and often displays the year as well. Most perpetual calendars can accurately calculate the date for 100 years or more – IWC claims a 570-year interval between adjustments! Perpetual calendar mechanisms are extremely complicated, functioning like a mechanical computer with the “data” entered by the gears and levers. It has to calculate true-or-false equations to properly advance the date and month. A special star-shaped gear with a tiny sub-gear attached automatically calculates the leap year cycle as well as the 30-31 day cycle, with provisions for odd years (leap years follow a formula that isn’t necessarily every 4 years – years divisible by 4 are considered leap years except when they are also divisible by 100, but again are considered if they are divisible by 400 – got that?). The depth of the teeth on the gear determine the number of days in the month; when a 30, 28 or 29 day month comes up the gear gathers up a pawl that instantly skips the requisite number of days when the date changes (remember that the date display always has 31 days and the short months need to skip the extra days mechanically). There are "simple" perpetual calendars (simple is a misnomer, even the most basic perpetual mechanism is highly complicated) that only calculate the 4 year cycle, but most true perpetuals nowadays have corrections for odd years. Being so complicated, perpetual calendars are priced accordingly – the most “affordable” perpetual calendars run in the 30 000$ range, and can easily reach into the $100 000 plus range.
Many calendar watches will often have a moonphase complication, something that has become a signature for classic and traditional designs. A moonphase disc displays the cycles of the moon through a crescent-shaped aperture on the dial. The moon rotates slowly, a fraction of a millimetre per day. A full lunar cycle is precisely 29.53 days. This causes problems when designing a complicated movement where precision is demanded. Because of that fractional day the lunar cycle will be slightly off after long periods of running, as it is impossible to gear the display for a precise correction (the normal design is to double the teeth on the display gear and have two moons printed on the disk. 59 teeth on the disc comes close to accurate when you divide it into two cycle of the moonphase). Many solutions have arisen; some are complex mechanism like in an annual or perpetual calendar but dedicated to the moonphase display only, others are a simple mechanical corrector activated by the user (the Jaeger LeCoultre Duomètre à Quantième Lunaire features a button dedicated to correcting the moonphase with a single push). The vast majority of moonphase displays feature no correction, however, and need to be adjusted every few months by the wearer. Not exactly a deal breaker by any means, but when you are building haute-horlogerie pieces it's common to find solutions to very minor problems in the pursuit of ever-more complicated and precise mechanisms.
The calendar is one of the most common mechanical complications available today, but that doesn’t mean it isn’t one of the more complicated. While a simple date is a relatively straightforward addition, a complex calendar with corrections for each month, the moonphase, and leap years is one of the most difficult complications to execute. Don’t take the humble date display for granted: it is a complication like any other, even if it isn’t as glamourous as a tourbillon or a minute repeater. And unlike those fancy displays of haute-horlogerie, the calendar is probably the most useful complication you can have in a watch.