The IWC Portugieser Eternal Calendar was awarded the Golden Hand Award at the 2024 Geneva Grand Prix for Watchmaking (GPHG). Featuring a calendar that displays time accurately up to 3999 years, as well as a moon phase display that deviates by only one day every 45 million years, this is arguably the most advanced calendar watch ever made. And what's more, its size is roughly the same as existing perpetual calendars. So how did the IWC development team create this masterpiece?
This eternal calendar automatically adjusts for leap years every 100 years. By improving an existing movement, IWC was able to mass-produce this mechanism. It is perhaps the most sophisticated calendar mechanism ever mounted in a wristwatch. Automatic winding (Cal. 52640). 54 jewels. 28,800 vph. Power reserve of approximately 7 days. Platinum case (diameter 44.4 mm, thickness 14.9 mm). Water resistant to 5 bar. Price upon request.
Photographs by Eiichi Okuyama, Yu Mitamura
Masamasa Hirota (this magazine), Yukiya Suzuki (this magazine): Interview and text
Text by Masayuki Hirota (Chronos-Japan),
Text & Edited by Yukiya Suzuki (Chronos-Japan)
Edited by Yuto Hosoda (Chronos-Japan)
[Article published in the July 2025 issue of Kronos Japan]
A simple solution to create the most sophisticated calendar watch ever made
The highlight of the watches announced in 2024 is likely to be IWC's Portugieser Eternal Calendar. This is not a perpetual calendar that automatically adjusts to leap years, but a secular calendar that eliminates the need for date adjustment every 100 years. While several watch manufacturers have produced secular calendars in the past, such as Patek Philippe's Caliber 89 and Franck Muller's Aeternitas, they are not very common. This is because they are extremely difficult to design and manufacture. In theory, this should be possible by adding a gear for adjustment every 100 years to a perpetual calendar, but incorporating this into the limited space of a wristwatch is extremely difficult. However, with this clever design, IWC has not only achieved a secular calendar and a precise moon phase that deviates by only one day every 45 million years, but has also succeeded in mass-producing them.
Before discussing this groundbreaking secular calendar, let's review the IWC perpetual calendar on which it was based. The perpetual calendar module developed by Kurt Klaus was fundamentally different from previous models. First, the power source. A typical perpetual calendar draws its power from the date wheel (24-hour indicator) that moves the hands. Klaus's perpetual calendar module, in contrast, was powered by the date ring, which moves once per day. In other words, a pin was set into the date ring, which advances once per day, and when the pin moved, all the linked calendars changed. This module was used in the 1985 Da Vinci Perpetual Calendar, which allowed all calendars to be advanced simply by turning the crown. Additionally, this perpetual calendar module, which did not use a date wheel as its power source, allowed for a simpler design. This had the advantage of being more resistant to shocks and less likely to break. This is why IWC later added perpetual calendar models to its sports watches, the GST and Pilot's Watch.
The Cal. 52000 series, which boasts a power reserve of approximately seven days thanks to its two barrels, is ideal as the base movement for a perpetual calendar. This time, major improvements have been made to this movement to make it a secular calendar. The balance spring is a flat balance spring rather than a winding balance spring. This is only a guess, but it may be a preparation for installing a silicon balance spring. The use of ceramics in the Pellaton automatic winding mechanism also makes it extremely durable.
The simple perpetual calendar also brought another benefit: extraordinary expandability. Even at the time of the Da Vinci, when it was equipped with the ETA7750, IWC's calendar module already had room for improvement. This was the reason for the four-digit year display at 8 o'clock and the precise moon phase display that deviated by only one day every 122 years. With the Portugieser, which has a larger base movement, this room for improvement was even greater. The result was a revolutionary moon phase mechanism that not only displayed the northern and southern hemispheres simultaneously, but also deviated by only one day every 577.5 years.
For many years, IWC had been planning to create a secular calendar, a watch with a superior compatibility to the perpetual calendar. It wouldn't be impossible to achieve with the space available in the Portugieser, and in fact, Kurt Klaus, who perfected the perpetual calendar, apparently had also expressed a desire to create a secular calendar. However, it was a young designer at IWC who brought the idea to life.
Essentially, all perpetual calendars, including IWC's, have a 48-month wheel that rotates once every four years to control the advance of the calendar days. The reason it rotates once every four years is to accommodate the end of February every four years. Normally, it would end on the 28th, but it needs to be advanced to the 29th. However, in a leap year, which occurs once every 100 years, the end of February ends on the 28th instead of the 29th. The secular calendar, or Portugieser Eternal Calendar, accommodates this.
This was made possible by a component called the "400-year gear," which rotates once every 400 years instead of four. This gear has three notches, and at the end of February every 100 years, one of the notches overlaps with a recess in the 48-month wheel, skipping leap years. As a result, the calendar will display accurately at least until the year 3999. Despite this impressive mechanism, only eight components were added to the existing perpetual calendar. "The 48-month wheel, which rotates once every four years, has five teeth. By adding a protrusion to one of these, we can add a 20-year wheel, which rotates once every 20 years. This 20-year wheel drives the 400-year gear," explains Stefan Ihnen, head of development. The reason for this extremely simple mechanism was the request to keep its size the same as the perpetual calendar. The development team developed a new calculation program and, after 2.7 trillion calculations, arrived at the simplest configuration.
Another distinctive feature is the moon phase display, which deviates by just one day every 45 million years! IWC's perpetual calendar was originally marketed as a moon phase display that deviated by just one day every 122 years. In the Portugieser, with its larger movement, this was refined to an accuracy of one day every 577.5 years, but this time, the accuracy has been increased to an incredible one day every 45 million years, rather than 4500 years. The incorporation of a reduction gear is the same as for the date change mechanism. IWC's development team simulated over 22 trillion possible combinations and came up with a new reduction gear train using three intermediate wheels. The more the speed is reduced, the more precise the moon phase display becomes, but it was unexpected that this could be achieved with just three intermediate wheels. However, Ihnen says that the lunar phase deviation can be further reduced. "The moon moves away from the Earth by about 3 to 4 cm every year, so we decided that there was no point in trying to refine it too precisely."
In theory, increasing the number of gears and teeth would make it possible to achieve a more precise moon phase display, even in a secular calendar. However, even though there is ample space, the Portugieser movement is only 38 mm in diameter. How on earth did they manage to reduce the number of gears? Ihnen attributes it to UV-LIGA. "Even if you could design it, it wouldn't be possible without precision parts. The gears that IWC has incorporated into the calendar and moon phase display are made with an extremely high level of precision. This would not have been possible with previous manufacturing methods, but thanks to LIGA we were able to optimize the tooth shape and size of the added gears."
Gears used in watches are generally made by pressing or cutting. However, in recent years, gears made using UV-LIGA have become popular. This involves electroforming a mold made using X-ray lithography, then plating it to create a part. While this is too extravagant for use in mechanical watches, in theory it is possible to create parts that are exactly the same as the blueprint. Furthermore, because the shape of the gears can be freely changed, it is also possible to give the teeth elasticity. In this case, IWC used UV-LIGA to create gears that are accurate to the micrometer, and to translate computer simulations into the real thing.
Through what could be described as obsessive pursuit, IWC has achieved an accurate calendar display up to the year 3999 and an ultra-high-precision moon phase. It is no surprise that this model won the Golden Hand Award at the 2024 Geneva Watchmaking Grand Prix. Upon receiving the award, CEO Christophe Grainger-Herr said, "When you look at this watch, it makes you think about what we as human beings should do." This is not just a watch that is precise, but one that also makes you think about the future of the Earth. Has such a timepiece ever existed?
