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Inside the Wristwatch, as You've Never Seen It Before

By Mark Bernardo - 2014-03-18T04:00:06Z

Photograph by Tom Schierlitz

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Perpetual Calendar

Unlike simpler calendar functions, which require users to manually reset the date for months with fewer than 31 days, a perpetual calendar anticipates the unequal lengths of all 12 months, including leap-year Februaries, decades into the future. Most—including the IWC Ingenieur Perpetual Calendar Digital Date-Month—need recalibrating only at the turn of each century. The innovation dates from 1898, when Patek Philippe put one in a ladies’ pendant watch, and works by means of a triple cam-and-lever system, which accounts for the seven months with 31 days, the four months with 30 days and the month of February—whose leap years are triggered by a separate disk that permits a 29th day every fourth revolution.