Devised to incorporate the specificities of the Gregorian calendar. It is perpetual because it automatically adjusts to months with 30 days and to the 28 or 29 days in February. For this, it incorporates a mechanical memory whose sequences are repeated every 48 months to correspond to the cycle of leap years. It must only be adjusted for the exceptionnally non-leap year years, the next being 2100 and 2400.
See Perpetual calendar watch and Watches with astronomical indications.