For the most part, calendar complications use the length of a day to show their information. If we know how long one day is we also know the length of months and years. Fortunately, the center wheel makes one rotation per hour. A reducing gear train on the dial side of the movement at a ratio of 24:1 gives us exactly one day, and enables the calendar complication to function. From the tall building metaphor, we now have two floors. The ground floor is the movement’s basic gear train, dividing time and transmitting power to the escapement. The second level is the calendar, using the center wheel arbor as its input.