Hindu calendar
The hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

s, as well as an Indian national calendar
Indian national calendar
The Indian national calendar is the official civil calendar in use in India. It is used, alongside the Gregorian calendar, by The Gazette of India, news broadcasts by All India Radio, and calendars and communications issued by the Government of India.The term may also ambiguously refer to the...

. Nepali calendar
Nepali calendar
The Nepali Calendar is a traditional solar calendar used in Nepal for both civil and religious purposes. The year begins in the month Baishakh; usually around 14 April....

, Bengali calendar
Bengali calendar
The Bengali calendar or Bangla calendar is a solar and sidereal Hindu calendar used by the Bengali people. It is used in the eastern Indian states of West Bengal, Assam and Tripura and in Bangladesh...

, Malayalam calendar
Malayalam calendar
Malayalam calendar is a solar and sidereal Hindu calendar used in Kerala, India. The origin of the calendar has been dated as 825 CE....

, Tamil calendar
Tamil calendar
The Tamil calendar is a solar and sidereal Hindu calendar used in Tamil Nadu. It is also used in Pondicherry , and by the Tamil population in Malaysia, Singapore, Mauritius and Sri Lanka. It is also used by Telugu speaking people in Tamil Nadu...

, Telugu calendar, Kannada calendar etc. are some prominent regional Hindu calendars.

Most of these calendars are inherited from a system first enunciated in Vedanga Jyotisha
Vedanga Jyotisha
The ' is an Indian text on Jyotisha, redacted by Lagadha .The text is foundational to the Vedanga discipline of Jyotisha. It is dated to the final centuries BCE...

of Lagadha, a late BCE adjunct to the Vedas
The Vedas are a large body of texts originating in ancient India. Composed in Vedic Sanskrit, the texts constitute the oldest layer of Sanskrit literature and the oldest scriptures of Hinduism....

, standardized in the Surya Siddhanta
Surya Siddhanta
The Surya Siddhanta is one of the earliest siddhanta in archeo-astronomy of the Hindus by an unknown author. It describes the archeo-astronomy theories, principles and methods of the ancient Hindus. This siddhanta is supposed to be the knowledge that the Sun god gave to an Asura called Maya. Asuras...

(3rd century CE) and subsequently reformed by astronomers such as Aryabhata
Aryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...

 (499 CE), Varahamihira
Varāhamihira , also called Varaha or Mihira, was an Indian astronomer, mathematician, and astrologer who lived in Ujjain...

 (6th c.