Modem applications for energy saving in buildings need to include daylighting. Natural illumination will not only reduce the demand of electrical energy for fixtures but will also reduce the energy demand of air conditioning during a hot period. Conventional approaches consider the natural illumination of architectural spaces by using a statically sky lumination distribution, e.g. the standard overcast sky according to the German DIN (DIN, 1983). This approach is not sufficient if energy saving should be calculated since it does not take the fluctuating sky lumination at the site of the building into account. This defect can be overcome by extend an existing illumination Simulation program that computes only statically by a dynamic routine.