Day-date
Two discs, one timing signal, and the movement’s peak torque demand
What it is
Day of the week and date shown together, most familiarly with the day in an arched window at twelve and the date at three. Two independent discs are driven from the same timing signal. This is real mechanical work rather than a layout choice.
History
Day and date had appeared together on wrist calendars since the 1940s, including Rolex's own references 8171 and 6062, where day and month sat in twin apertures and the date ran on a peripheral pointer. The 1956 Rolex Day-Date, references 6510 and 6511, fixed the modern format: full-word day at twelve, numeric date at three, on a self-winding Oyster in precious metal only. The mechanism is covered by Swiss patent CH322341A of 1955, credited to Marc Huguenin. Early production had trouble with date advance, and reference 6611 of 1957 brought the improved calibre 1055. The layout has since been copied so widely that "day-date" is now a generic name for it.
How it works
The hour wheel drives a reduction gear down to a twenty-four-hour wheel, which carries fingers that advance both the seven-position day star and the thirty-one-tooth date ring at the same point in the cycle. Each disc has its own jumper spring holding it registered under its aperture. Where the change is instantaneous, the driving finger loads a spring through the late evening and trips it at midnight so both discs jump. Because the date ring is large and the day disc heavy, both changing at once is the peak torque demand in the whole movement, which is why quickset correction is designed to be locked out during the change window: forcing a correction while the finger is engaged is what breaks calendar work.
Parts required
A day star and a thirty-one-tooth date ring, a twenty-four-hour driving wheel carrying advance fingers for both, two jumper springs, and the quickset correction wheels engaged from the crown.
What makes it difficult
Two masses have to move at the same instant, from the same mainspring, at the moment the movement has least to give. That is why so many day-date calibres drag the change over a period rather than jumping it, and why the crown is locked out of correction near midnight.
In the catalog
Related
- Day of the week: A seven-position disc advanced once every twenty-four hours
- Date: The most common watch complication; and the most corrected
- Annual calendar: Knows 30- vs. 31-day months; needs one correction per year