On 29 February 2024, a rare event occurs: an extra day is added to the calendar, signaling a leap year. Since 2001, A. Lange & Söhne has released nine timepieces to commemorate this occasion. However, 2024 is special as it showcases the Saxon manufacture’s groundbreaking approach to the perpetual calendar mechanism.

In a leap year like 2024, the difference between the perpetual calendar and the annual calendar becomes apparent. The anticipation begins on New Year’s Eve, as the leap-year indication switches from 3 to 4 at midnight. It takes 59 days until the perpetual calendar can demonstrate its full capability. It will correctly transition from 28 to 29 February, but then switches directly to 1 March at midnight, presenting a technical challenge requiring a mechanical program that maps the different durations of all 48 months across the entire four-year cycle.

The Classic Approach
Traditionally, this task is handled by a program wheel with 48 notches and steps corresponding to the different durations of the 48 months in the four-year cycle. While the wheel rotates only once every four years, the notches and steps are sampled by a lever. The deeper the notch, the sooner the mechanism switches to the first day of the next month. The deepest notches correspond to the four Februaries, with one slightly shallower marking the leap year February. The mechanism accounts for the different lengths of the months for an entire century, requiring corrections only in the secular years 2100, 2200, and 2300.

Seven out of nine A. Lange & Söhne timepieces with a perpetual calendar and other complications rely on this traditional principle. However, the LANGE 1 TOURBILLON PERPETUAL CALENDAR and the LANGE 1 PERPETUAL CALENDAR are based on a completely new approach.

The Innovative Approach
Innovating in a perpetual calendar is challenging due to its traditional roots dating back to the mid-1700s. Undeterred, A. Lange & Söhne set out on a new path. The development of the LANGE 1 TOURBILLON PERPETUAL CALENDAR launched in 2012 introduced a patented peripheral month ring, a new type of month display that replaces the traditional mechanism. This solution posed new challenges, including finding a way to rotate the large ring instantaneously by 30 degrees while switching to the new month.

The month ring is driven via its internal gearing, rotating around its own axis once a year. A spring-loaded sampler lever glides along a circumferential contour with wavy recesses inside the gear rim. The more it is deflected, the shorter the month. In February, an extender of the sampler lever contacts a cam beneath the leap-year disc, indicating whether it is a common year with 28 days in February or a leap year with 29 days. This innovative approach is also used in the LANGE 1 PERPETUAL CALENDAR introduced in 2021.