Propitiously enough, Āryabhaṭa himself gives a clue to his date in his Āryabhaṭīya, Kālakriyāpāda, verse 10, which reads:
ṣaṣṭyabdānām ṣaṣtir yadā
vyatitās trayaś ca yugapādāḥ ।
tryadhikā viṃśatir abdās
tadeha mama jammmo ’titāḥ ॥
"When sixty times sixty years and three quarter yugas (of the current yuga) had elapsed, twenty three years had then passed since my birth."
The stanza states that at the time of composing the work 60 X 60 =3600 years of the Kaliyuga had elapsed and, incidentally, the author adds that is was then 23 years after his birth. The date works out to the end of the Kali year 3600, corresponding to the Śaka year 421, the date being March 21, 499. The position of the equinoxes were then zero degree and so the mean positions of the planets would be accurate if computed using their parameters given by the author in the Gītikāpāda of the Āryabhaṭīya. (3)
For computation of the mean planetary positions for future dates, a small correction was to be applied, which Āryabhaṭa’s follower Lallācārya gives in his work Śiṣyadhivṛddhida. (4)
While most commentators follow the above interpretation, in Kerala, the Parahita system enunciated by Haridatta in 689 AD interprets the verse to mean that the date given in the verse is the date of birth of Āryabhaṭa and not the date of the composition of the work. "When sixty times sixty years and three yugas had elapsed, twenty three years of my age have passed since then."
This would mean that Āryabhaṭa was born in Kali 3600, equivalent to Śaka 421 or AD 499, and that he composed the Āryabhaṭīya when he was 25 years old, i.e. in Śaka 444 or AD 522. But the parameters of Āryabhaṭīya accord only with the year Kali 3600. For this reason, in the Parahita system corrections for computing the mean longitudes of planets using Āryabhaṭa’s parameters but with the epoch of Śaka 444 have been evolved for use in astronomical computations. (5)