This Patek Philippe Perpetual Calendar reference 3940J is one of the defining modern perpetual calendars, prized for the balance of its display and the restraint of its proportions, a watch many collectors still regard as the reference point for the complication at Patek Philippe. Its self-winding caliber 240 Q with micro-rotor drives the perpetual calendar and moonphase with the clarity that made this model such a lasting benchmark. The 18k yellow gold case measures 36mm across and 8.8mm thick, notably slim for the complication and composed enough to disappear under a cuff when needed. A silver dial keeps the watch as calm as its architecture deserves. In the 3940, Patek Philippe made a perpetual calendar that feels complete, and that completeness still defines its appeal.