A date window is a small opening cut in a watch dial. Through it, you read the date printed on a disc that turns beneath it. It is one of two ways a simple calendar watch shows the date. The same kind of window can also show the day of the week or the moon phase.
How does a date window work?
A date window is a type of aperture, which is a hole cut in the dial. Beneath it sits a disc printed with the days of the month. As the disc turns, one number at a time lines up with the opening, and that is the date you read. A simple calendar watch does not have to use a window. It can use a hand that points to a day number from 1 to 31 instead. Both methods give the same information.
Where it comes from
The Rolex Datejust is credited with making the date window a common, mainstream feature. It is described as the first self-winding watch with a date window at 3 o’clock that changed by itself at midnight. Rolex later added a magnifying lens over the window, called the cyclops. It sits on top and enlarges the small printed number, so the date is easier to read at a glance.
Not to be confused with
A big date display is not simply a larger date window. Building one takes more than cutting a wider opening and printing bigger numbers. A single-disc date window shows the whole number on one disc. The Lange 1 big date works differently. It splits the digits across two discs, one for the tens and one for the units. Together, the two discs make up the date.
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