Hindu calendar is a collective name for most of the luni-sidereal calendars and sidereal calendars traditionally used in Hinduism.
The Hindu calendars have undergone many changes in the process of regionalisation. Some of the more prominent national and regional Hindu calendars include the official Nepali calendar in the himalayan country Nepal and in India Punjabi calendar, Bengali calendar, Odia calendar, Malayalam calendar, Kannada panchanga, Tulu calendar, Tamil calendar, Vikrama Samvat and Shalivahana calendar in the Deccan states of Karnataka, Telangana, Maharashtra and Andhra Pradesh.
The common feature of many regional Hindu calendars is that the names of the twelve months are the same. 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 derived from Gupta era astronomy as developed 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).