Located at “12 o’clock” on the dial, IWC’s annual calendar shows the month, date and day in three separate windows. The mechanism automatically takes into account the differing lengths of individual months. Unlike a perpetual calendar, however, the annual calendar is unable to make allowance for the leap days in February or the leap years. Once a year, then, at the end of February, it requires manual correction. In the case of IWC’s annual calendar, the correction is made simply, via the crown. The hour wheel sets in motion the date advance wheel, which is equipped with two different advance fingers. One of the fingers is responsible for the date and month displays, the other for the weekday. Every night, towards midnight, the date is moved forward with the help of the upper advance finger. Two pins on the date disc control the month display and the programme wheel that defines the various lengths of the months. The first of these pins advances the programme wheel by one division. A feeler recognizes 30-day months on the advance lever. The feeler registers a deeper insert on the programme wheel and makes more space available for the advance. The following day, the advance lever moves the date forward by 2 days.