An instantaneous date is a date display whose number switches to the next day in a split second, right at midnight. It is the opposite of a dragging change, where the disc creeps forward over a stretch of time. Of the three main types of calendar mechanism, it is regarded as the most sophisticated.
How does an instantaneous date work?
The mechanism stores energy in a spring and releases it at midnight. During the final minutes before midnight, the 24-hour wheel gradually tensions this accumulating spring. At exactly midnight, a cam lets the spring go. Its stored energy passes straight to the finger on the driving wheel, which advances the date in one leap. Winding and releasing the spring takes noticeably more energy than simpler systems need. The mechanism appears mostly in more expensive watches, where an exact display is prized.
Not to be confused with
Calendar mechanisms come in three main types: direct drive, semi-instantaneous and instantaneous. The names sound alike, but they behave differently. A semi-instantaneous date changes in two separate stages rather than a single jump. A dragging change is different again: the hand or disc creeps forward gradually over a stretch of time. In horology, a display counts as instantaneous when its indicator leaps almost at once from one value to the next.
Where it comes from
The Tissot Visodate was introduced in 1953 to mark Tissot’s centenary. It is described as the first automatic watch whose date window changed instantly at midnight. Rolex followed in 1955, when it gave the Datejust an instantaneous midnight date change. Until then, the date on the original Datejust had changed over slowly.
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