Nicephoros Gregoras (1295-1360) is considered along with his teacher, Theodoros Metochites, as the most significant scholar and the greatest astronomer of Byzantium. Gregoras was the first to propose, in 1324, a correction to the calculation of Easter, and the reform of the Julian calendar similar to the one adopted later, in 1582, by the Pope Gregory XIII. His famous book "Byzantine history" is an important source for the researches on medieval Serbia. He was also in a diplomatic mission on the court of Serbian king Stefan Dečanski.