A perpetual calendar is a calendar mechanism, found in clocks and watches, that automatically displays the correct date throughout the year, allowing for months of different lengths and for leap years without manual correction. The term also describes simple desk calendars with rotating dials or cards that can be reset to show any date.
Mechanical perpetual calendars require a complex train of wheels and cams, typically including a wheel that turns once every four years to account for leap years. They were developed by skilled horologists in the eighteenth century and appear in fine regulators, astronomical clocks and complicated pocket watches, often alongside moon phase and day-of-the-week indications. French and American makers produced calendar clocks in larger numbers in the nineteenth century, and the complication was later popularised in Swiss wristwatches. Desk perpetual calendars in brass, silver, wood or enamel were popular from the Victorian period onwards. When buying a clock or watch with this feature, check that the calendar advances correctly and that all indications are working, as repairs to these mechanisms require specialist skill. Many simpler examples are not truly perpetual and need adjustment at the end of short months.
