Wikipedia biography of aryabhatta the greatest
Following the Ganitapada, the next section is the "Kalakriya" or "The Reckoning of Time." In it, Aryabhata divides up days, months, and years according to the movement of celestial bodies. He divides up history astronomically; it is from this exposition that a date of AD has been calculated for the compilation of the Aryabhatiya. [4].