Bhāskara II
Hindu calendars were refined during the Gupta era astronomy by Āryabhaṭa and Varāhamihira in the 5th to 6th century. These, in turn, were based in the astronomical tradition of "Vedāṅga Jyotiṣa", which in the preceding centuries had been standardised in a number of (non-extant) works known as "Sūrya Siddhānta". Regional diversification took place in the medieval period. The astronomical foundations were further developed in the medieval period, notably by Bhāskara II (12th century).