This is the date display on your mount. The date can be a simple calendar, or a perpetual calendar. For a simple calendar, the date must be set when the month does not include 31 days. The perpetual calendar, on the other hand, allows the date to be displayed by taking into account all the days of the month, as well as leap years.
At Charlie Paris, all our men's watches have a calendar. Besides being a useful complication, the calendar actually makes your wrist look better. We also have the Initial Calendar template, which will give you the date as well as the day and month.