Let’s first explain the difference between an annual calendar and a perpetual calendar. Every wristwatch with an annual calendar displays the date, often also the month, and sometimes the day of the week. It indicates all of these correctly every day of the year, except at the end of February. Every annual calendar needs to be adjusted to go from February 28/29 to March 1st.
The perpetual calendar does not have to be adjusted at all. It automatically switches from February 28/29 to March 1st and automatically corrects for leap years. That is, until the next secular year, which will be in 2100.
The new Quadriennium does not have to be adjusted at the end of February, at least in three out of every four years. This means it automatically takes into account that February has 28 days (three out of four years) and jumps to March 1st. Only in the leap year will it need to be adjusted, because the calendar’s mechanism “thinks” that February always has 28 days. Essentially, the Audemars Piguet Millenary Quadriennium is the only watch that needs to be adjusted backwards in February.
The balance wheel, which is held in place by a large bridge that is beautifully finished by hand, is part of the famous Audemars Piguet escapement. This proprietary AP escapement, with its double balance spring, was originally launched in 2006 inside the Millenary Tradition d’Excellence No. 5, and works without lubrication thanks to its improved geometry. The double hairspring compensates for potential poising flaws, and should improve the watch’s chronometry.
Take a moment to watch the video below, which perfectly explains how the AP escapement differs from a regular Swiss anchor escapement.
source: watchtime.com By Frank Geelen