A calendar is a common complication for watches and clocks.
There are various methods for displaying the calendar
Though many watches place the date aperture at 3 00, such windows or pointer sub-dials can be found anywhere on the watch face.
Some watches feature a so-called big date complication, allowing the display of single digits on multiple wheels for a more readable display of the date.
The most common implementation is a simple calendar with a date and (optionally) day wheel. These simple mechanisms advance the day and date wheel once a day at midnight. Such simple calendars advance 31 times before returning to “1”, requiring manual advancement at the end of most months.
A watch that adds the name of the month is a so-called full calendar or annual calendar complication. Nearly all such watches automatically adjust to the varying length of each month, giving 29 days to February. Thus they only require manual adjustment once per year, except on leap years.
The perpetual calendar watch keeps track of leap years as well. Some also account for the skipping of leap years in future centuries.