At the same time, this cam is the centrepiece of the mechanical calendar programme. Running round its edge is a series of raised and indented sections, which provide information about the differing lengths of the months. It functions in a way similar to the punch cards from the days when computer technology was in its infancy. To ensure that the leap years are also part of the equation, the cam represents a full four-year cycle consisting of 48 months. A single recess – the one for the 29th of February – is, therefore, deeper than all the others.
In the shorter months, another mechanism comes into play. An additional click on the date advance lever rests on an eccentric that is connected directly to the date wheel. At the end of months with fewer than 31 days, it drops down from the eccentric to rest on a projection. “During the switching sequence, which takes place in the middle of the night, it advances all the days before the non-existent 31st of the month before the normal pawl comes into play and advances the date wheel by a single tooth,” says Klaus, describing one of the essential features of his design.
This additional mechanism is controlled indirectly by the month cam. In months with fewer than 31 days, a feeler arm connected to the date advance lever falls into a recess. The deeper the recess, the greater the radius through which the date advance lever moves. A long radius causes the additional pawl to be retracted slightly further and drops from the eccentric at the end of the month. “The protrusions and recesses on the month cam determine the different radii and whether and when the additional pawl comes into action,” says Klaus.