The quickset is a convenience function, not a precision one: it serves to advance the date disc without dragging the hands through hours of useless rotation. I've seen it added to or absent from apparently identical watches, and that difference is often worth far more than the market knows how to price it.
The mechanical principle is simple. The crown has an intermediate position — distinct from the hour-setting position — that disconnects the normal transmission system and commands the date disc directly through a click mechanism. Each click of the crown advances the date by one day. In the most common calibers, like the Rolex 3035 and later the 3135, this is achieved in the first crown position. In the Zodiac Sea Wolf 72B the system works differently: you press the crown inward and the date jumps upon release, a less elegant but effective solution.
The distinction between single and double quickset is relevant in the Day-Date: the ref. 1803 has neither; the 18038 has only the date quickset; the 18238 has double quickset, which acts independently on both date and day of the week. It's not a catalog detail — anyone who adjusts a Day-Date without double quickset knows how much time is lost with each month change.
The technical problem with quickset is the so-called danger zone. In the hours before and after midnight — roughly from 10 p.m. to 2 a.m., with variations from caliber to caliber — the date hammer is already engaged with the wheel teeth but without enough tension to complete the jump. Forcing the quickset during that window strips the teeth. I've seen it many times on the bench: an intervention that could have been avoided. The NOMOS DUW 6101 solves the problem with a dual-level friction system that eliminates tension buildup, making quickset safe at any time of day and in both directions. It's one of the few truly original solutions to this problem as old as the cyclic date itself.
Bidirectional adjustment — forward and backward — is rare. The NOMOS Tangente Neomatik 41 Update and the DUW 6101 offer it. The majority of calibers, including those in the ETA 2824 family and equivalent Sellita movements, work only forward: to correct a date you must turn all the way to the next day. It's not a serious flaw, but it is a real limitation.
A practical note: the quickset does not change the date synchronously with the natural change at midnight. After a manual correction, the watch resumes its cycle from where the internal calendar mechanism finds itself. In some calibers, an adjustment made in the late afternoon can advance the midnight change by a few hours. It's worth keeping in mind.