Perpetual calendar
A watch complication that displays the date and accounts for all calendar irregularities automatically. A properly-set perpetual calendar requires no manual correction until the year 2100.
A perpetual calendar is a watch complication that displays the date and accounts for all calendar irregularities (months with 30 days, 31 days, February's 28 days, and leap years) automatically. A properly-set perpetual calendar requires no manual correction until the year 2100.
What it shows
Most perpetual calendars display:
- Day of week
- Date (1-31)
- Month
- Year (or a leap-year indicator on a sub-dial)
Higher-end versions add:
- Moonphase
- Equation of time
- Solar and sidereal time
- Hijri calendar correspondence
How it knows about leap years
A four-year cam tracks the leap-year cycle. February shortens to 28 days in normal years and 29 in leap years automatically. The Gregorian rule (years divisible by 100 are not leap years, except those divisible by 400) is why most perpetual calendars need a manual correction in 2100, which is divisible by 100 but not 400.
Secular perpetual calendars
A handful of secular perpetual calendars (Patek Philippe 5208P, Greubel Forsey QP à Équation, Jaeger-LeCoultre Master Hybris Mechanica) account for the 100-year rule mechanically. They will not need correction until 2400.
Setting and maintenance
A perpetual calendar must remain running. If it stops for more than 24 hours, all displays drift out of sync and need re-setting via dedicated pushers, often requiring a professional. Some brands include a quick-set system that simplifies the procedure; others require manual advancement through every missed day.
Famous perpetual calendars
Patek Philippe 5320G, 5230G, Reference 1518 (vintage), Audemars Piguet Royal Oak Perpetual Calendar 26574, Vacheron Constantin Patrimony QP, A. Lange & Söhne 1815 Annual Calendar, IWC Da Vinci Perpetual.
Related: moonphase, tourbillon, patek-philippe.