Hanne Darboven recorded dates and numbers according to a self invented system. She often referred to her work as "mathematical literature". Dedicated to the structure of time and memory the starting point for her large-scale serial works are often special dates and historical events. The divisions of the calendar as the conceptual basis of her work are typical. Each sheet from the exhibited series "The Sundial/ The Moonlight" dated 1976 refers to the first two months of the year1976. On each sheet are two dates. For each date the check sum is being calculated, and visualized in the length of the line below. 1976 was a leap year. The additional day was added in order to match the human calendar with the natural, i.e. the tropical calendar. This connection between the natural and calendrical process, between 'Sunlight' and 'Moonlight' on the one side and the calendar on the other is the central topic of this work.