Perpetual calendars are the ultimate expressions of date complications. When driven under constant power, it stands up to extended scrutiny, taking into account larger passages of time, notably of leap years. Unlike an annual calendar, which needs resetting once a year at the end of February, a perpetual calendar only requires a correction after every full century when the Gregorian calendar omits a leap year.
This anomaly was the result of Pope Gregory XIII’s reform in 1582, which reduced the margin of error under the Julian calendar by omitting a leap year three times in the course of every four centuries, with an average year comprising 365.2425 days instead of 365.25 days. As such, centurial years are only leap years if they are divisible by 400. By design, a regular perpetual calendar, however, will interpret the years 2100, 2200, and 2300 as leap years when in fact they are common years (at least in all parts of the world that had adopted the Gregorian calendar), thus requiring manual adjustment by one day on March 1 of these years.
As such, a superior form of perpetual calendar was birthed to accommodate this nuance — the secular perpetual calendar. This mechanism can accurately account for three consecutive centurial years that are non-leap years in a 400-year cycle.
Mechanically, as movements are designed to keep track of larger passages of time, from seconds and minutes, which are directly driven by the gear train, to 100- and 400-year cycles, derived from the humble motion works of a watch, they evolve in complexity, with a number of increasingly elaborate mechanisms in between to compensate for variances in
