An aperture or window is a small opening in a dial, face, or case.
The most common aperture is the date window used to display the date, often at 3 00 on the dial. Apertures are also used for other calendar functions, including day, month, year, and so on on perpetual calendar watches. They can be used for GMT functions, rising hours, and other complications. Apertures can even reveal the movement through the face or case as in the case of open heart or skeleton designs.