Calibre 89 © Patek Philippe
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. Unlike the Perpetual calendar it is also automatically adjusted for the exceptionnally non-leap year years, the next being 2100 et 2200.
See Watches with astronomical indications.