What does “Perpetual Calendar” mean?

A perpetual calendar automatically shows the correct date across months of different lengths and leap years, so it shouldn't need correcting until the year 2100, which is not a leap year in the Gregorian calendar. It usually displays the date, day, month and leap-year cycle, often with a moon phase. Collectors often call it a QP, from the French 'quantième perpétuel'. Patek Philippe made one of the first perpetual calendar wristwatches in 1925.

Perpetual calendars are among the most respected complications, with long traditions at houses such as Patek Philippe, Audemars Piguet and Vacheron Constantin. They need care: if one stops, setting it correctly can take time, and some can be damaged by adjustments at the wrong time of day. Buyers should check that every indication advances correctly and that the leap-year display matches the date. Servicing by a qualified watchmaker is costly, so recent service records add value.

Example: 'Perpetual calendar, all indications correct, serviced 2022.'