DATE INDICATION
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The date indicator represents the day of the month (a consecutive numbering from 1 to 31).
Time is inseparable from astronomy. Since the Egyptians (and likely even earlier), time measurements have been based on astronomical observations, with all time-related information closely linked to them. Even before the introduction of the Gregorian calendar in 1582, clocks commonly displayed various astronomical data. The Astronomical Clock of Prague, built in 1410, is undoubtedly one of the most renowned examples.
Astronomically, the date indicator represents the average interval between two instances of the sun passing the zenith at a specific location on Earth. It is incorrect to consider the duration of a day (24 hours) as merely the time it takes for the Earth to complete one rotation around its axis. A full rotation of the Earth (360°) takes an average of 23 hours, 56 minutes, and 4 seconds. However, the Earth also revolves around the sun (an astronomical year). Due to this dual movement, the Earth must rotate slightly more than a full turn around its axis (approximately 361°) for the sun to reappear at the zenith of a given point on Earth.
Technically, a date indicator is not hard to make. Its value changes at regular intervals (every 24 hours) when the hour hand completes two rotations around the dial. The date is commonly displayed by a disc or a hand, with the disc being the most typical display method.
Most of the time, the date indication disc occupies the circumference of the movement. This provides the dual advantage of allowing more space for its mechanism (usually placed under the dial) and displaying the date more legibly. The date is then shown in an aperture in the dial (most often at 3 o’clock). The disc is numbered from 1 to 31. At midnight, the time display mechanism drives the date indicator, causing the next value to appear in the window. Throughout the month, the values progress from 1 to 31, after which the cycle restarts. This system does not account for months with 28, 29, or 30 days. Consequently, on the first day of months following those with 28, 29, or 30 days, the watch user must adjust the day indicator. Therefore, five adjustments are needed annually (on the 1st of March, May, July, October, and December).
When displayed by a hand, the principle is the same. At midnight, the date indication hand (often positioned at the centre of the movement) jumps to the next value. Numerous alternatives exist in the display method and positioning. Sometimes, the data indicator is off-centre. In such cases, the display becomes smaller, reducing its legibility.
Different driving systems exist and can be classified into two main families: semi-instantaneous mechanisms and instantaneous mechanisms.
In the first system, the disc or the star is gradually driven (in a trailing manner) by the finger of the driving 24-hour wheel. The jumper then snaps over the next tooth of the disc or star, positioning the disc or hand instantly into its next position.
The instantaneous mechanism involves gradually arming a spring during the final minutes before midnight. At midnight, the spring’s energy is released instantaneously. The spring then drives the finger of the driving wheel swiftly, causing the instant jump of the date or day disc or hand. The instantaneous mechanism prevents any reading confusion in the minutes following midnight. However, this system requires more energy than the semi-instantaneous system.
In most cases, the date is indicated by a disc displaying the current date in an aperture or by a hand on an auxiliary dial (subdial). In such instances, the date display can also be of the retrograde type.