Audemars Piguet kickstarted its 150th anniversary with a bang, debuting the brand-new ultra-thin perpetual calendar Caliber 7138 in both the Royal Oak Ref. 26674 and the Code 11.59 by Audemars Piguet. The movement is outstanding, both technically and philosophically, not as an exercise in complication for its own sake, but as a complication conceived in service of the wearer.

The problem with perpetual calendars is that while they are feats of mechanical ingenuity, they are impractical in daily life. Setting one is a chore. Adjustments usually require multiple recessed pushers scattered around the case, each operated with a stylus or pin tool. Make a mistake, like setting the calendar backwards or during the dead zone, and there’s a risk of damaging the mechanism. In short, they are brilliant on paper, beautiful on the wrist, but frustrating to live with and have been this way pretty much since they were transposed to the wrist in 1925. There have been improvements over the years, but often at the expense of the familiar visual language that has come to define the complication and none have enabled individual adjustment of each indicator via the crown.

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar in Steel (Image: ©Revolution)

What Audemars Piguet has done with the Caliber 7138 is confront those long-standing limitations, giving the wearer full control over six separate indications – all through a three-position crown. In the neutral (first) position, the crown winds the movement. Pulling the crown out to the second position allows the date to be set clockwise and the month counterclockwise. When the crown is pulled all the way out to the third position, the hands can be set to the correct time. What’s remarkable is that when the crown is pushed from the third position back into the second position, the day can be set by turning the crown clockwise, and the moon phase, by turning the crown counterclockwise.

The Five Patents

The self-winding Calibre 7138 measures 29.6mm x 4.1mm and is based on the superb Calibre 7121 first introduced in “Jumbo” Royal Oak ref. 16202 in 2022 to mark the 50th anniversary of the Royal Oak. The long-running Caliber 5134 is slightly thicker at 4.3mm while the 5133 in the RD#2 is a mere 2.89mm high, both of which have highly unusual program wheels and both were based on the Caliber 2120, which remains the slimmest full-rotor automatic movement to this day. Power reserve is 55 hours in the new movement and it runs at 28,800 vph in 41 jewels.

The Calibre 7138 uses the sophisticated base Calibre 7121. The balance assembly was optimized for stability, incorporating a free-sprung design with a broad, flat rim and recessed weights to reduce air resistance, while a higher 4 Hz beat rate improved shock resistance, all of which is supported by a full balance bridge.

The perpetual calendar mechanism in the Calibre 7138 was developed over a period of five years and is protected by five patents. The first two patents cover the ingenious ultra-thin solutions originally introduced in the Caliber 5133, developed for RD#2.

The ultra-thin calendar mechanism in the Calibre 7138

In a traditional perpetual calendar, the 48-month program wheel is not a gear in the conventional sense as it does not have teeth. Instead, it functions as a cam, encoding the logic of different month lengths over a four-year period through notches and raised sections of varying depths. As such, a regular 48-month toothed wheel, along with an auxiliary train, is typically required to relay information to the month display. But in the Caliber 7138, the 48-month program wheel is both a cam and a gear. It simultaneously interacts with the grand lever and drives the month pinion directly.

On the left is the date star and on the right, the 48-month program wheel with a leap year indicator wheel pinned to it

The date wheel, on the other hand, features a single asymmetrical tooth adjacent to a notch, which allows it to be engaged by a secondary pawl for multi-step advancement at the end of months with fewer than 31 days. One of the standard teeth also projects radially beyond the others, acting as a finger that advances the month wheel once per full revolution via a series of intermediate wheels. This design replaces both the end-of-month snail cam that interacts with the grand lever and the finger that advances the month wheel, effectively reducing three layers to one.

As a result of this, the grand lever is more compact. There are two pawl fingers mounted on it. The first pawl finger advances the date star by one day everyday. The second pawl finger is used only at the end of months with fewer than 31 days to skip over the “excess” dates (like the 29th, 30th, or 31st) and jump to the 1st of the next month.

On a normal day, the first pawl finger pushes a regular tooth to advance the date by one day. But at the end of a short month, the feeler tail detects (via the cam slots on the program wheel) how many days to skip, and the lever swings a bit further. That extra swing allows the second pawl finger to drop into the notch behind the asymmetrical tooth. Then, during the same motion, the second pawl pushes the special tooth multiple steps forward, skipping over the excess days in one motion. After that, the first pawl finger still moves the date forward one final step so the mechanism lands on the 1st of the next month.

The use of a grand lever, however, makes any backward adjustment of any date indication impossible. But it remains significantly more convenient than earlier crown-controlled, gear-based perpetual calendars especially in scenarios where you need to cycle through multiple months. In those systems, adjusting the calendar often requires advancing the date repeatedly just to move the month forward. By contrast, in the 7138, all six indications can be adjusted directly.

The third patent addresses a problem with traditional hand-based date displays. In a such a display, the hand moves in constant angular steps (equal pitch), since it is driven by regular gears. However, the date numerals vary in width. To compensate, the spaces between numbers are made unequal, so the hand always moves the same distance, but this results in a visually uneven and compressed dial, especially around the 20s and 30s and particularly between 31 and 1. The date wheel in the Calibre 7138 inverts this logic. Rather than altering the spacing between numerals which compromises the display, it varies the motion of the hand to match the actual visual width of each numeral. The date wheel has teeth of different widths. Narrower teeth for single-digit numerals and wider teeth for double-digit numerals. The lever pushes one tooth per day, but the angular distance covered varies depending on the tooth width.

A closer look at the date wheel with teeth of different widths

The fourth patent covers the manual correction of the date and month. In a traditional perpetual calendar, the date and month correctors are separate buttons recessed into the side of the case. They pose a risk to water-resistance and require a tool to operate, as well as add mechanical complexity such as additional springs and levers because they must first disengage the grand lever from the calendar cams before making any correction.

Vaguely visible beneath 48-month program wheel is a four-arm month-correcting cam. When the crown is rotated counterclockwise in the second position, it engages a gear train that rotates this cam and one of its arms lifts the grand lever away from the 48-month program wheel to prevent interference or damage. As the cam continues to rotate, another arm engages a month-correcting pinion, which in turn rotates the month pinion and updates the display. Isolation and correction therefore occur in a single continuous motion.

Second position – month correction

When the crown is rotated clockwise in the same position, it instead drives a separate cam, this time dedicated to date correction. A small driving stud on the edge of the date-correcting cam contacts the grand lever and pivots it away from both the 48-month cam and the date star, again isolating the system to avoid mechanical conflict. As the cam continues its rotation, the grand lever is pushed further until the spring-loaded pawl mounted on it advances the date star by one position. Once the correction is made, the cam moves past the lever, allowing it to fall back into its resting position under the force of its return spring, re-engaging with the 48-month cam and resuming normal operation.

Second position – date correction

The last patent covers the all-in-one crown, specifically how the same crown, using only three axial positions (fully pushed in, intermediate, and fully pulled out), can control and actuate multiple independent functions in a movement. The innovation lies in how the mechanism interprets how the crown arrives at the intermediate position, not just the fact that it is there.

Third position – time setting

The clever part is that this second, intermediate position serves two entirely different sets of functions, depending on whether the stem arrives there from a pulling action from first to second position or a pushing action (from third to second position). This directionality is sensed and mechanically translated by a pull-out piece, which has a pin that rides against a cam fixed to an actuating lever. This cam is shaped asymmetrically with inclined planes on both sides so that the direction of axial motion determines which plane the pin slides against, and therefore which way the actuating lever pivots.

The actuating lever is essentially the heart of the selector. It pivots about a fixed axis and carries four wheels – two on the left and two on the right. When the lever tilts one way, after a pull from first to second position, it brings the wheels on one side into engagement for adjusting the date and month. When the lever tilts the other way after a push from third to second, it brings the wheels on the other side into engagement for the day and moon phase. So even though the stem ends up in the same axial spot, what happens when you rotate it from there depends entirely on the direction you came from.

Royal Oak Perpetual Calendar Ref. 26674 in Steel and Sand gold

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar in Sand Gold (Image: ©Revolution)

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar in Steel (Image: ©Revolution)

The Royal Oak Perpetual Calendar Ref. 26674 measures 41mm by 9.5mm, matching the dimensions of its predecessor, the Ref. 26574. It is offered in stainless steel or AP’s proprietary Sand Gold, a new alloy that debuted in the Royal Oak Tourbillon Openworked last year. It sits somewhere between white gold and rose gold, with a muted sandy-beige tone that feels more neutral and modern in character. Depending on lighting, it may appear silvery, champagne, or even faintly pink, but never as distinctly red as rose gold. The Sand Gold with a matching dial is stunning but there’s a particular satisfaction in seeing this extraordinary movement in the most emblematic Royal Oak configuration – steel with a blue dial.

The dial layout appears to be similar to its predecessors at first glance but has been subtly revised. The moon phase remains at 6 o’clock and the day at 9 o’clock, but the date and month have swapped positions. Notably, the day subdial now incorporates a co-axial 24-hour counter, which highlights the no-correction period between 8 PM and 4 AM. However, even if an attempt is made to adjust the calendar during this window, the mechanism is protected.

All told, this is a no-brainer if you can land one. Headline-grabbing innovations have their place; they dazzle, push boundaries, capture imaginations and drive study. But the innovations that hit home are those that improve the daily experience of the wearer beyond intellectual appreciation. These are the quiet revolutions that deepen our connection to the watch and turn ownership into something more intuitive, more enjoyable and ultimately more enduring.

Tech Specs

Audemars Piguet Royal Oak Selfwinding Perpetual Calendar 41mm

Reference: 26674ST.OO.1320ST.01; 26674SG.OO.1320SG.01
Movement: Self-winding Caliber 7138; 55-hour power reserve
Functions: Hours and minutes; central seconds; perpetual calendar with week indication; day; date; astronomical moon; month; leap year
Case: 41mm × 9.5mm; stainless steel or 18K sand gold; water-resistant to 50m
Dial: Grand Tapisserie pattern in blue or sand gold
Strap: Stainless steel or 18K sand gold bracelet; both with AP folding clasp
Availability: Regular production but a special “Anniversary” limited edition version is available in 150 pieces each
Price:CHF 88,000 in stainless steel (CHF 91,000 for the anniversary edition), and CHF 130,000 in sand gold (CHF 133,000 for the anniversary edition)