This paper describes an algorithm for calculating the photoperiod length (daylength as intercepted by plants) with special emphasis on its use in models describing plant growth and development. The ability to calculate simply and exactly the length of the day for different locations is essential not only for the models describing photoperiodic effects on plant development but also for the simulation of the daily sum of the produced photosynthates or the calculation of the average daily temperature. Two previously published algorithms for calculation of photoperiod length were compared with ours, based on the system of equations describing movement of earth around the sun. The curve gained when plotting daylengths calculated by our algorithm against a particular date is asymmetrical with respect to daylength in equinoxes, while the curve showing the data calculated by both compared algorithms is apparently symmetrical. The differences between our algorithm and the other two algorithms increase with increasing latitude: at 50 degrees N they are in the range of 6.9-13.8 min. We have also demonstrated an effect of location height, geographical longitude and a year on calculated daylength of a particular day.