Day-date

The day of the week is a different question from the date, and for most people the more urgent one. It governs whether an office is open, when a delivery is due, when a market trades. Calendar watches of the 1930s and 1940s showed both, but generally cut the day to three letters because there was no room to spell it out. Rolex’s Day-Date of 1956 fixed the form the complication has kept since.

Mechanically the day is the simpler indication, because the week has no irregularities. The same twenty-four-hour wheel that drives the date carries a finger which, once per revolution, advances a seven-toothed star bearing the names of the days, where a jumper holds it. Since the week never varies in length, the day needs no correction as long as the watch runs. In an instantaneous construction both displays are driven by energy stored in a cam and released within a second or two of midnight.

The second disc costs height and dial area, and it doubles the load released at midnight, so the demand on the mainspring at the change is greater than for a date alone. The curved aperture of the spelled-out type needs a large disc with its printing arranged on an arc, which is part of why the complication was slow to reach smaller cases.