Annual calendar

An annual calendar sits one step below a perpetual-calendar mechanism. Patek Philippe describes its own version in exactly those terms: "It indicates the date for months of 30 and 31 days and needs only one correction per year, at the end of February." February is the one month such a mechanism cannot resolve, because it has 28 or 29 days while the wheel governing month length distinguishes only 30 from 31.

A European patent shows how the mechanism is arranged. EP 0 756 217 A1, "Annual calendar mechanism for timepieces", describes a date ring driven one step a day together with "an annular crown wheel(8), partially superposed on the date ring(1), which rotates once a year", carrying "5 interior teeth(10) in positions corresponding to 31 day months". At the end of a shorter month a second finger acts on one of those teeth and jumps the ring on to the first of the next month. The specification ends: "A manual correction is required for the month of February."

The open question is who invented it. Patek Philippe's page says the annual calendar was "invented and patented by Patek Philippe" and introduced in 1996, but gives no patent number. The only annual-calendar patent of that period this Codex could retrieve and read, EP 0 756 217 A1, was filed on 15 July 1996 with a priority date of 28 July 1995 by Compagnie des Montres Longines Francillon SA, inventor Frank Vaucher. Patek Philippe's claim is recorded here as the company's claim about itself; its own patent has not been read, so the attribution is left unresolved.