Hindu calendar is a collective term for the various lunisolar calendars traditionally used in India. They adopt a similar underlying concept for timekeeping, but differ in their relative emphasis to moon cycle or the sun cycle and the names of months and when they consider the New Year to start. Of the various regional calendars, the most studied and known Hindu calendars are the Vikram Samvat (Bikrami) found in northern, western and central regions of the Indian subcontinent,the Shaka calendar used in the Deccan region, Tamil calendar used in the south, and the Bengali calendar used in the east – all of which emphasize the lunar cycle, their new year starts in spring, with their heritage dating back to 1st millennium BCE. In contrast, in regions such as Kerala, the solar cycle is emphasized and this is called the Malayalam calendar, their new year starts in autumn, and these have origins in the second half of the 1st millennium CE. A Hindu calendar is sometimes referred to as Panchanga (पञ्चाङ्ग).
The ancient Hindu calendar is similar in conceptual design to the Jewish calendar, but different from the Gregorian calendar. Unlike Gregorian calendar which adds additional days to lunar month to adjust for the mismatch between twelve lunar cycles (354 lunar days) and nearly 365 solar days, the Hindu calendar maintains the integrity of the lunar month, but insert an extra full month by complex rules, every few years, to ensure that the festivals and crop-related rituals fall in the appropriate season.
The Hindu calendars have been in use in the Indian subcontinent since ancient times, and remains in use by the Hindus in India and Nepal particularly to set the Hindu festival dates such as Holi, Maha Shivaratri, Vaisakhi, Raksha Bandhan, Pongal, Onam, Krishna Janmashtami, Durga Puja, Ramlila, Vishu and Diwali. Early Buddhist communities of India adopted the ancient Indian calendar, later Vikrami calendar and then local Buddhist calendars. Buddhist festivals continue to be scheduled according to a lunar system. 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. Similarly, the ancient Jainism traditions have followed the same lunisolar system as the Hindu calendar for festivals, texts and inscriptions. However, the Buddhist and Jaina timekeeping systems have attempted to use the Buddha and the Mahavira lifetimes as the reference point.
The Hindu calendar is also important to the practice of Hindu astrology and zodiac system, most of which it adopted from Greece, in centuries after the arrival of Alexander the Great. The Indian national calendar or "Saka calendar" was redesigned in an effort that started in 1952 based on the traditional Hindu calendars, and it was adopted on March 22, 1957.
[The current year] minus one,
multiplied by twelve,
multiplied by two,
added to the elapsed [half months of current year],
increased by two for every sixty [in the sun],
is the quantity of half-months (syzygies).
— Rigveda Jyotisha-vedanga 4 Translator: Kim Plofker
Time keeping was important to Vedic rituals, and Jyotisha was the Vedic era field of tracking and predicting the movements of astronomical bodies in order to keep time, in order to fix the day and time of these rituals. This study was one of the six ancient Vedangas, or ancillary science connected with the Vedas – the scriptures of Hinduism. The ancient Indian culture developed a sophisticated time keeping methodology and calendars for Vedic rituals.
David Pingree has proposed that the field of timekeeping in Jyotisha may have been "derived from Mesopotamia during the Achaemenid period", but Yukio Ohashi considers this proposal as "definitely wrong". Ohashi states that this Vedanga field developed from actual astronomical studies in ancient India. The texts of Vedic Jyotisha sciences were translated into the Chinese language in the 2nd and 3rd centuries CE, and the Rigvedic passages on astronomy are found in the works of
Zhu Jiangyan and Zhi Qian.
Timekeeping as well as the nature of solar and moon movements are mentioned in Vedic texts. For example, Kaushitaki Brahmana chapter 19.3 mentions the shift in the relative location of the sun towards north for 6 months, and south for 6 months.