A "perpetual calendar" watch gives the date, day, month and usually moon phases while automatically taking the number of days in the month, and the cycle of leap years, into account. It is a masterpiece of architecture and miniaturization.

With its months lasting 31, 30 or 28 days, and with 29 days in February every fourth year, our Gregorian calendar sets quite a challenge to watchmakers who must reproduce these variations in a mechanical movement. The majority of calendar watches feature simple calendars which have to be manually corrected five times a year, after each month with fewer than 31 days. Compare these with the perpetual calendar, one of the culminations of the watchmakers art whose origins go back to the end of the eighteenth century. It will continue to show the correct date by adjusting to the variable number of days in the month and by taking the cycle of leap years into account. To accomplish this exploit, the movement draws on a "mechanical memory" of 1,461 days or four years. Most perpetual calendar mechanisms use a differential gear mechanism from the hour wheel and can comprise several hundred wheels, gears, levers and other parts.

The perpetual calendar remains a classic among complications, yet never ceases to inspire new interpretations and perfecting. It lends itself to ever-slimmer versions and incorporates correction systems that are sophisticated yet instinctive. Meanwhile, watch designers rise to the creative challenge opened up by its multiple indications date, day, month, moon phases, leap year, even year, experimenting with the different display modes - hands, apertures, retrograde, jumping - to compose dials that range from truly classic to truly original.

Despite its name, the perpetual calendar will have to be corrected by one day on March 1st, 2100. Not through any fault of the mechanism but because of a quirk of the Gregorian calendar which rules that this year, like three century years out of four, will not be a leap year. Only a few rare "secular calendar watches" take this particularity into account. Watchmakers have also developed "annual calendars" which recognise the length of each month except for February, and therefore need only be corrected once in the year.

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