AUDEMARS PIGUET, 18K OPEN-FACE PERPETUAL CALENDAR MOON PHASE POCKET WATCH, REF. 5555BA

The traditional perpetual calendar indicates the date, the day of the week and the months. Unlike a standard calendar watch, the perpetual calendar mechanism ingeniously takes into account the months’ unequal lengths of either 28, 30 or 31 days and the intercalary day of February 29th in a leap-year without requiring manual adjustment. In fact, assuming the watch is kept running, adjustment is only required once per century for three centuries in a row out of every 400 years. This is because in the Gregorian calendar which is a 400 year cycle, the leap-year is omitted three times every 400 years.