It’s a grail of a complication: A perpetual calendar, capable of adjusting forward or backwards through time, over days, weeks, months, years and leap years, knowing the solstices, equinoxes, and the difference between sidereal and mean solar time without the need to re-adjust again with a pusher. Since the earliest scientific endeavor to place mechanical certainty on the nature of time (think of the Antikythera mechanism), these mechanics have always been complex. However, Greubel Forsey, with it’s “Quantième Perpétuel à Équation,” seems to have found the solution; and, in turn, has also quite brilliantly reinvented the complication.

There are two problems facing the mechanical operation of a perpetual calendar. The first lies in the earth’s idiosyncratic rotation on its axis and around the sun; there’s no exact number of days in a full orbit of the earth. The problem is complicated further by the composition of the modern day calendar, with its leap years and variable number of days in the month. The second issue for a perpetual calendar is modern-day travel, which can take the watch wearer forwards or backwards by hours or even days; and even across the international date line.

In the past, watchmakers have traditionally favored a modular calendar plate, one advantage of which is that the calendar mechanism (already difficult to master) can be built up alongside the base movement and use an existing movement caliber, thus avoiding designing and building a completely new movement in order to integrate the calendar system. In pocket watches that featured perpetual calendars, the subdials driven by this mechanism were still reasonably large and easy to read. Once the sub dials were miniaturized into the wristwatch, however, these subdials became smaller and more difficult to read.

The Greubel Forsey Quantième Perpétuel à Équation surpasses these technical issues in a single bound. There are no additional pushers, no self-winding system, no mass of different dials to read. The watch has one crown, manual winding, and a linear read-out across the bottom right section of the dial. In effect, this small research-driven watch firm from La Chaux-de-Fonds has produced a mechanical day-date analogue computer that can realize any day of the week, date, and month, both forward and backward in time, while accounting for leap years and monthly idiosyncrasies.

Perpetual calendars usually require almost constant running and adjustment, hence most are fitted with an automatic winding mechanism. If the watch can use the wrist’s energy to make sure that the watch does not stop, then the calendar should keep moving with the days, months, and even years. If the watch stops, then re-adjustment takes time and requires the use of an additional pusher or two. Greubel Forsey wanted to do something different, something that could be adjusted irrespective of when the watch stops. They thought of an alternative solution: a manual wind, swift adjustment with two speeds: one for the day-date and one for the time of day (hours and minutes). Hence, the Greubel Forsey Quantième Perpétuel à Équation comes with a rapid adjustment (for the days, months, and years) and a simple adjustment (pulling out the crown) for the hours and minutes. In one watch the company has solved the problem of the need for an automatic winding mechanism, and eliminated the time needed to (laboriously) adjust the calendar if the watch stops. Another plus: the “crown can be turned in either direction to adjust all indications, without the risk of damaging the mechanism,” explains Forsey.

The genesis of this breakthrough can be seen in Robert Greubel and Stephen Forsey’s backgrounds. Both began their careers in the watch restoration and repair business: Greubel at his family’s atelier in Alsace, Forsey at Asprey’s in London. “Robert and I were inspired to create the QP à Équation partly by the systems used in the large astronomical clocks found in the 15th century but also because we wanted to bring to our collectors an easier way to wear a perpetual calendar complication. This is why we designed this timepiece so that all eight indications (seasons, equinoxes, solstice, equation of time, date, day, month and year) could be controlled through the one winding crown at 3 o’clock, says Forsey. However, it took two years to produce a working

prototype as part of the Experimental Watch Technology (EWT) workshop. Shown to a few journalists at Baselworld back in 2006, the EWT piece was modular, with the module parked on top of a manual wind movement and the calendar a set of concentric dials. With the working prototype in place, the refinement process started—and proved more complex than anticipated.

The first order of business was legibility of the dial. Both Robert and Stephen wanted a linear readout, which presented a number of problems. First, a number of disks must all turn simultaneously as the minute and hour hands traverse the midnight hour. Second, the day and date disks will turn every 24 hours, but at the end of each month the date may have to jump anywhere from one to four days. The date in conjunction with the day and month disk will turn irregularly throughout the year, and then again every leap year. Human variability in determining the number of days in a month (somewhere between 28 and 31) coupled with the irregularity of the earth’s orbit around the sun (not 365 days but approximately 365 and a quarter) requires that the watch be able to adjust itself, unless you wish to include the use of a pusher.

The solution to the problem was a new form of perpetual calendar, one for which Greubel Forsey has a number of patents pending, and one that owes a great deal to the spherical differential developed by Forsey’s knowledge of vintage Bentley gearbox construction. A central set of cogs and levers within the watch serve as its “mechanical computer” and runs the entire perpetual calendar. It’s pure genius.

Almost all other perpetual calendars run on a series of cogs and levers that are arrayed laterally across the diameter of the movement. As the watch hands pass midnight, a grand lever (usually secured below 12) allows the program wheel to move on a set number of days, depending on the month, over a 4-year or 48-month cycle. However, these watches required pushers to adjust the days of the week, which proved to be a problem every time.

Think of it this way: You are at home on Monday, February 28 of a non-leap year. At midnight, your watch should switch to Tuesday, March 1. The day has only moved one step; the month has also moved one step, but the date has moved 4 days (if 31 days per month is your default). To add to the complexity, if this happened to be a leap year, the same pattern would be required of the day and month, but the date would now move forward 3 days. It’s the tying together of the date and days across the months that usually requires the use of pushers.