A secret holiday for owners of a perpetual calendar watch is 29 February. This is the date on which the Gregorian calendar gives us an extra day. The reason: the earth does not orbit the sun precisely once every 365 days. It takes 365 days, 5 hours, 48 minutes and 45 seconds. As a result, we are missing nearly a fourth of a day every year. To avoid Christmas eventually being celebrated in the summer at some point, every four years, an extra day offsets the difference between the solar year and the calendar year.
Once again this year, timepieces with perpetual calendars can demonstrate their capabilities. In two respects, actually: they must not only correctly transition from 28 to 29 February but then also jump directly to 1 March at the end of the day. This may sound simple, but it is a formidable technical challenge. It requires the development of a mechanical programme that maps the different durations of all 48 months across the entire four-year cycle. As a rule, this task is handled by a wheel with 48 teeth in which the durations each month during a four-year period are permanently stored in the form of gaps that have different depths.
With the LANGE 1 TOURBILLON PERPETUAL CALENDAR, A. Lange & Söhne’s product developers chose a totally different approach. They placed the month indication on a large circumferential ring that rotates about its centre axis once a year in twelve 30-degree steps. The display is endowed with recesses of different depths for the individual month durations as well as with a complex sampling mechanism that also takes leap years into account.