Besides the precise time, it is above all the date that is important for planning and scheduling events. That's why the date can be found on many watches in a window positioned at 3, 4 or 6 o’clock. It provides a view on the date disc that turns below the dial. A large date display, as for example on A. Lange & Söhne’s iconic Lange 1, has an even more complicated construction. The two digits are placed on two separate discs that advance synchronously and form together the current date.
A weekday display provides even more orientation in time. Both indications are often placed next to each other at 3 o’clock. A classic example of this watch type, the Rolex Day-Date launched in 1956, however, boasts the day of the week written out in full at 12 o’clock. The full calendar offers a further increase in comfort. In addition to the weekday and the date, it also displays the current month. In some models this task is carried out by a hand rotating around the dial instead of by a window-type display.
Whereas minutes, hours, days, and weeks have always the same length and can thus be reproduced by regularly shaped toothed wheels, the months of our calendar system represent a real challenge for watchmakers due to their irregularly varying length. The date display requires therefore in most watches manual correction five times per year. At the end of each short month the date display has to be switched forward by a day. This is not necessary with so-called annual calendar watches. The construction and careful assembly of these watches require a high degree of watchmaking expertise. The gear train of the Annual Calendar from Wempe Glashütte takes into account the different lengths of the months. The alternating switch of the date display on the 30 or 31 to the first day of the next month is controlled by a cam disc. However, this complicated mechanism requires a tiny correction once per year, at the end of February. It doesn’t take into account the varying length of this month.
There are only very few movements that master the automatic switch from February 28 to March 1 – and even do not fail in leap years: every four years they grant February one additional day. Such a complication taking into account many cycles of different length is called a “perpetual calendar”. This title is not an exaggeration. Moon-phase displays as the one adorning Reference 5320G from Patek Philippe will only have to be adjusted by one day after 122 years and all perpetual calendar watches will only need a little correction in 2100 because there will be no leap year: a task to be assigned with joy to the proud heirs of this sophisticated mechanical masterpieces.