As an alternative to peering hard at the miniscule date window in a wristwatch, a big date display (also often called “large date”, or “grande date”) is an obviously good idea, but constructing one is not as straightforward as cutting a bigger date window and painting larger numbers. The simplest date display is a ring with the numerals 1 to 31 printed on it, with teeth running along the inner edge of the ring. At midnight, a gear indexes the date wheel, advancing it one day, and the date display is thus updated.
A big date display, on the other hand, brings a whole new level of complexity to the proceedings. You could, perhaps in a very large watch, construct an XL-size date ring, which would indeed make for a bigger date display. But in a watch of, say, 38mm in diameter, the whole date ring becomes too big for the watch!
Rolex introduced an ingenious hack in 1954 by affixing a Cyclops magnifier to the crystal of its Datejust, enlarging the date display by 2.5 times. But a true mechanical solution to this problem had to wait till 1994, at the re-launch of A. Lange & Söhne after a hiatus of more than 40 years, by Walter Lange and Günter Blümlein.
The Lange 1, one of the four watches first introduced by A. Lange & Söhne at its relaunch, was the first example of a big date display being used in a wristwatch. To fit the large numerals into the 38.5mm case, it was necessary to use two discs: one for the tens and another for the ones. The critical moment in the operation of a big date watch comes on the changeover from the 31st of one month to the 1st of the next. Here, the ones disc has to remain stationary on the changeover, and the tens disc has to switch over to zero. That’s in addition to the problem of coordinating the switchover of the tens and ones discs during the change from the 9th to the 10th, and the 19th to the 20th.
The solution then was the patented A. Lange & Söhne mechanism with two discs, which is still used exclusively by the brand today. Each disc of the mechanism has its own “program wheel” that indexes the two discs — the circular ones disc, and the cruciform tens disc — individually, in order to coordinate the changeover correctly. The program wheels are, in turn, both indexed by a single central date wheel, whose teeth pass those of the ones date wheel on the switchover from the 31st to the 1st, rather than indexing them. This is achieved through using teeth of varying length on the program wheel for the ones date disc.
Elaborately constructed jeweled jumper springs hold the various wheels and discs in place until the appropriate moment for the switchover occurs; the entire mechanism can be adjusted via a pusher at 10 o’clock, and the date switches instantaneously as well.
The Lange big date mechanism alone adds a whopping 66 components to the watch. Consider that a simple, non-instantaneous date wheel (common in older watches which do not allow quickset date adjustment from the crown) adds just three parts – the date wheel, a gear that rotates once every 24 hours to index the date wheel, and a jumper spring to hold the date wheel in place – and you begin to understand why A. Lange & Söhne is so justifiably proud of its big date mechanism introduced in 1994.