The calendar has, across the gulf of centuries, given our species a sense of the big picture, of dailiness and eventfulness as well as a vital historical consciousness. By contrast, time measured by the clock seems to be abstracted from the motion of the universe, as if time was granular or a series of short, disjointed phases rather than continuously hastening the frantic world on with each tick.
In our modern technological age, the calendar serves as a connection to the natural world, whereby the rhythms of the earth and sky coordinate with that of human life. Naturally, a mechanical watch that denotes the progress of the calendar has to embody these cosmic rhythms in gear patterns and ratios, namely the speed at which the Earth rotates on its own axis, forming the day, the Moon’s orbit around the Earth, giving us the month, and finally, the Earth’s orbit around the Sun, which marks a year.
The highest order of such watches is the perpetual calendar as it needs to accomplish a high degree of automation, namely a yearly reiteration, hence having to obey the dictates of the Gregorian calendar in which each leap year has 366 days instead of 365, giving February 29 days instead of the common 28. This means that the mechanism has to correctly care for all irregularities of the calendar, acting at the end of each month, changing itself to the first day of the following month, sometimes after 30 days and other times after 31 days, changing once a year on the 28th day of February and finally, extending February by a day every four years. The complexity of encoding this nest of rules makes the gear train of a perpetual calendar one of the most intricate, perhaps second only to that of a minute repeater — both of which, with much irony, are driven by the humble motion works that spell out the time in every watch.
However, a perpetual calendar, contrary to the optimism expressed in its name, is not everlasting. By design, a perpetual calendar omits the one rule that makes a calendar Gregorian rather than Julian and that is, centurial years are only leap years if they are divisible by 400.
