Calendar is one of the most popular watch complications. There are three of them: tourbillon, repeater and perpetual calendar.
The widespread type of the complication is a so-called "Casino window", named by alternating black and red numbers on the dial.Big Date
As you might guess from the name, this is the most readable display. Here the window and a figure showing the date is much bigger than traditional indicators. There is a version of this complication with two windows: the left one displays the numbers from 0 to 3, and the numbers in the right one go from 0 to 9.Pointer Date
The central arrow or a crescent moon, indicating the date along the outer edge of the dial. This pointer is also sometimes called «Bankers».Subsidiary Dial
The date can be displayed on a so-called sub-dial, which is most often used with other complications.Day-Date
Day-Date function among other indications displays day of the week. Options are usually set with the crown; its movement in one direction changes a day, in the other direction - the date. Adjustments can also be made with a small button on the side of the case using a special tool called "stylus".
Triple calendar is also called "full calendar" and displays not only the date but also the day of the week and the month.Perpetual calendar
This is one of the most difficult complications that makes a timepiece equipped with this function to be very rare and expensive. It accurately displays the date, day, month and year, and even takes into account leap years. This calendar will require adjustments only in 2100, when a leap year is going to be omitted.Annual calendar
The annual calendar is an intermediate type of complication between a perpetual calendar and a triple calendar. It is not programmed to take into account leap years and if not corrected manually every year it will be counting until February 31st.