Abstract: Several transparent elements showing periodically-varying curves and a transparent element showing a date scale are superposed over each other and a ground element which shows a number of year scales of different length, subdivided from 0 to 100 years. The transparent elements are to be adjusted in respect to their position to said year scales according to a reference date and a desired reading date. The correct position of all curves and their inter-relationship are immediately visible i.e. without any foregoing use of the known mathematical tabulations which are standard e.g. in the biological rhythm field.