The perpetual calendar is a more complicated version of the annual calendar. It also displays the day, date and month, and will automatically correct for the months with 30 or 31 days. Additionally, a perpetual calendar will also keep track of leap years meaning it will know whether February has 28 or 29 days and display it accordingly. If set correctly and kept running the perpetual calendar, as its name suggests, will continue to produce the correct calendar date until the year 2100 which represents an anomaly in the gregorian calendar system.
The way this complication works is by a complex cam which acts as the ‘brain’ of the calendar display. In combination with a sequence of levers, the watch will automatically skip the 31st of the month if necessary and more importantly adjust for the month of February. Although the tourbillon is often seen as haute horlogerie, in actual fact, the perpetual calendar movement is vastly more difficult to produce. Some say it is second only to the pinnacle of movements in watchmaking – the minute repeater.