Aryabhatiya, an improved work, is product of mature intellect , which he wrote when he was 23 years old. Unlike in the Aryabhata siddhanta , the civil days are reckoned from one sunrise to the next, a practice which is still prevalent among the followers of Hindu calendar.
He gives a clue to his date of birth in his Aryabhatiya... The date works out to the end of the Kali year 3600, corresponding to the Saka year 421, the date being 21 March 499 ...and that he composed the Aryabha?ya when he was 25 years old, i.e. in Saka 444 or AD 522. Page 4
His fame rests mainly on his Aryabhatiya, but from the writings of Varahamihira (Sixth century AD), Bhaskara I, and Brahmagupta (seventh century) it is clear that earlier he composed the Aryabhata Siddhantha (voluminous) is not extant. It is also called Ardharatrika Siddhanta , because in it the civil days were reckoned from one midnight to the next; 34 verses on astronomical instruments from this have been quoted by Ramakrishna Aradya.
He was the father of the Indian cyclic astronomy which resulted in the planetary theory that determines more accurately the true positions and distances of the planets (including the Sun and the Moon )...was also the first to produce celestial latitudes...proposed the scientific cause of eclipses as against the mythological demon Rahu [Moon's node]. His ideas resulted in the new school of Indian Astronomy – the Aryapasha ((Aryabhata School) based on the text of Aryabhatiya.