Mostly, these are inherited from a system first enunciated in Jyotish Vedanga (one of the six adjuncts to the Vedas, 12th to 14th century BC/BCE), standardized in the Surya Siddhanta (3rd century) and subsequently reformed by astronomers such as Aryabhata (499), Varahamihira (6th century), and Bhaskara (12th century). There are differences and regional variations abound in these computations, but the following is a general overview.