The Annual CalendarPatek Philippe invented the annual calendar in 1996, creating one of the most successful complications in the brand's history. The annual calendar reads the length of every month automatically, correcting itself for 30- versus 31-day months without any input from the wearer. The one exception is February. At the end of February, the owner advances the calendar manually, usually in the first days of March. One intervention per year, in a single predictable window.The practical case over a perpetual calendar is real. A perpetual calendar is a more complex mechanism, requiring no corrections at all (except for the century years), and as a result costs significantly more to buy and service. If you put it in a drawer for a month and come back to it, you reset everything from scratch, just as you would with any watch. The annual calendar splits the difference: simpler to set than a perpetual and far less demanding than a standard calendar over the course of a year.