Big date
Two elements instead of one ring, and a Dresden opera-house clock behind it
What it is
A date display roughly two to three times the area of a standard window, made by splitting the number across two elements instead of printing it on one ring. You see two digits in one large aperture, often with a thin dividing bar between them. It adds real parts and real work over a single date ring.
History
The display idea comes from the five-minute clock built for Dresden's Royal Court Theatre, opened in 1841, by court clockmaker Johann Christian Friedrich Gutkaes with his employee Ferdinand Adolph Lange, who later founded A. Lange & Sohne. That clock shows the time in two apertures, Roman hours on the left and five-minute intervals on the right, and it still hangs above the stage of the rebuilt Semper Opera House. A. Lange & Sohne revived the layout as a date on 24 October 1994, when Walter Lange presented the Lange 1, using a patented arrangement of a units ring and a tens cross. Lange is very widely credited with the first big date in a wristwatch, but that credit traces largely to the brand and brand-adjacent sources; what is solidly documented is that Lange patented and popularised the twin-element outsize date in 1994 and that it became the reference implementation. Glashutte Original followed with its Panorama Date, distinguished by having no dividing bar between the digits.
How it works
Instead of one ring printed 1 to 31, a big date uses two elements: a units disc carrying 0 to 9 and a tens element carrying 1, 2, 3 and a blank. Each has its own program or drive wheel so the two can be indexed independently, because the units element must advance every day but hold still at the 31-to-1 rollover while the tens element steps only every ten days. Both are held by jumper springs and driven from the twenty-four-hour wheel.
Parts required
A units disc and a separate tens disc or cross, a dedicated program or drive wheel and jumper spring for each, and a reinforced date-advance train able to move both at once.
What makes it difficult
Torque and inertia: two masses to move instead of one, at a single instant. That is why some big-date movements change over a period rather than instantaneously, and why some designs require manual correction at month end.