The Calendar disc of the watch displays the date and in some cases the weekday, month, and year. However, the regular calendar watches do not take into account different lengths of the months making the user need to reset the date 5 times a year when the month does not end on the 31st . The Perpetual calendar automatically make adjustments for different lengths of each month as well as leap years until the year 2100. Usually the perpetual calendar shows the date, month, day of the week and in most cases the moonphase. These type of watches carry big mechanical complications making these watches highly desirable luxury items.

Breitling Bentley Mark VI Perpetual Calendar