Making a timepiece with a perpetual calendar and equation of time display that’s easy to set was no simple feat. To do so was a technical challenge that required the team at Greubel Forsey to create a so-called “Mechanical Computer.”

Using a bi-directional crown with a push-button selector on the end of the crown and a dial indicator to confirm which mode the watch is in (that toggles between time and perpetual calendar setting modes), the wearer can set both the perpetual calendar and the time, with ease. The equation of time, therefore, is automatically set based on the date given by the integrated perpetual calendar, which displays not only the difference between apparent and mean time (on the multi-color caseback indicator), but also the month, season, equinoxes, and solstices.

To achieve this level of sophistication, Greubel Forsey designed “A stack of cams with movable fingers shift the indications on the dial and caseback of the timepiece and the complete mechanism is fully integrated within the movement. The month’s cam changes the month, displayed in a window on the front and also moves the Equation of Time disc on the back. The years’ cam controls the leap year indication on the front and also the millesime and seasons on the back. The development of this coding device not only overturns the conventional way of setting the indications but also displays them simultaneously on both dial and caseback.”

Simply put, this is one of the most technically advanced mechanical calendar wristwatches ever developed. So it’s not surprising that when it was originally introduced in 2017, Greubel Forsey took home the GPHP Calendar Watch Prize the same year.

As far as time, you have the hours and minutes displayed via centrally mounted hands, small seconds are shown in a subdial at 8 o’clock, and there’s 24-seconds tourbillon located in between 9 and 11 o’clock. With regard to the calendar function, there’s the perpetual calendar display on the front, showing day, big date, and month in a single subdial located between 4 and 5 o’clock — along with an adjacent leap year indicator. Then there’s a 24-hour day-night indicator shown via a small aperture along the peripheral of the dial, in between 7 and 8 o’clock. And finally, on the dial side, there’s the 72-hour power reserve display.

Flip the watch over, and you’ll see the so-called “millesime” function, which is a year disc. Lastly, the most complicated element, the mechanical computer — is the ultra-complicated Equation of Time display which indicates the difference between apparent solar and mean time (from 0-16 minutes), as well as the months (letters), seasons (colors), equinoxes (semi-circles), and solstices (circles).

All of this is achieved in a movement measuring 36.40 mm x 9.60 mm and that consists of 624 total components, with 75 jewels (including Olive-domed jewels) and that measures 36.40 mm x 9.60 mm. The requisite housing measures a relatively conservative 43.50 mm x 16 mm — and for this particular 2020 variant features a rich chocolate-colored dial that perfectly pairs with the 18K red gold hands, markers, and case.