IWC's perpetual calendar watches work with the utmost precision: in 577.5 years, the display will diverge by just one day from the moon’s actual course. The perpetual calendar displays the date, month and day of the week on three subdials of the same colour at “3”, “6” and “9 o’clock”, respectively. A small window in the bottom left-hand section of the dial reveals the four-digit year display. Mechanically programmed, the calendar takes into account the different length of the months and even the leap years. Nevertheless, every 100 years (2100, 2200, etc.) a leap day normally due is omitted, which means that a watchmaker will have to advance the calendar manually on 1 March. In 2300, another intervention will be necessary when the current century slide with the figures 20, 21 and 22 will need to be replaced with a new one for the years 2300 to 2599.