Hindu calendar is a collective name for most of the luni-sidereal calendars and sidereal calendars used in India since ancient times. It has undergone many changes in the process of regionalization, and today there are several regional Hindu calendars. It has also been standardized as the Indian national calendar. Some of the more prominent regional Hindu calendars include the Nepali calendar, Assamese calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan States of Karnataka, Telangana, Maharashtra and Andhra Pradesh. The common feature of all regional Hindu calendars is that the names of the twelve months are the same (because the names are based in Sanskrit) though the spelling and pronunciation have come to vary slightly from region to region over thousands of years. The month which starts the year also varies from region to region. The Buddhist calendar and the traditional lunisolar calendars of Cambodia, Laos, Myanmar, Sri Lanka and Thailand are also based on an older version of the Hindu calendar. Most of the Hindu calendars are inherited from a system first enunciated in Vedāṅga Jyotiṣa of Lagadha, a late BC adjunct to the Vedas, standardized in the Sūrya Siddhānta (3rd century) and subsequently reformed by astronomers such as Āryabhaṭa (AD 499), Varāhamihira (6th century) and Bhāskara II (12th century). Differences and regional variations abound in these computations, but the following is a general overview of the Hindu lunisolar calendar.