Vikram Samvat (IAST: Vikrama Samvat) (abbreviated as V.S. (or VS) or B.S. (or BS)); Listen (help·info)) (also called the Vikrami calendar) is the historical Hindu calendar from the Indian subcontinent and the official calendar of modern-day India and Nepal. It uses lunar months and solar sidereal years.
The Vikram Samvat is notable because many ancient and medieval era inscriptions use it. It is said to be named after the legendary king Vikramaditya, but the term "Vikrama Samvat" does not appear in the historical records before the 9th century, rather the same calendaring system is found by other names such as Krita and Malava. In the colonial era scholarship, the era was believed to be based on the commemoration of King Vikramaditya expelling the Sakas from Ujjain. However, later epigraphical evidence and scholarship suggest that this theory has no historical basis and very likely was an error. Starting in the 9th century and thereafter, epigraphical artwork uses Vikrama-Samvat, suggesting that sometime around the 9th-century, the Hindu calendar era that was already in use became popular as Vikram Samvat, while Buddhist and Jain epigraphy continued to use an era based on the Buddha or the Mahavira.
Kalakacharya Kathanaka ("An account of the monk Kalakacharya") by the Jain sage Mahesarasuri gives the following account: Gandharvasena, the then-powerful king of Ujjain, abducted a nun called Sarasvati, who was the sister of the monk. The enraged monk sought the help of the Śaka ruler King Sahi in Sistan. Despite heavy odds but aided by miracles, the Śaka king defeated Gandharvasena and made him a captive. Sarasvati was repatriated, although Gandharvasena himself was forgiven. The defeated king retired to the forest, where he was killed by a tiger. His son, Vikramaditya, being brought up in the forest, had to rule from Pratishthana (modern Paithan in Maharashtra). Later on, Vikramaditya invaded Ujjain and drove away from the Śakas. To commemorate this event, he started a new era called the "Vikrama era". The Ujjain calendar started around 58–56 BCE, and the subsequent Shaka era calendar was started in 78 CE at Pratishthana.
The association of the era beginning in 57 BCE with Vikramaditya is not found in any source before the 9th century CE. The earlier sources call this era by various names, including Kṛṭa (343 CE and 371 CE), Kritaa (404 CE), the era of the Malava tribe (424 CE), or simply, Samvat.
The earliest known inscription that calls the era "Vikrama" is from 842 CE. This inscription of Chauhana ruler Chandamahasena was found at Dholpur, and is dated Vikrama Samvat 898, Vaishakha Shukla 2, Chanda (16 April 842 CE). The earliest known inscription that associates this era with a king called Vikramaditya is dated 971 CE. The earliest literary work that connects the era to Vikramaditya is Subhashita-Ratna-Sandoha (993-994 CE) by the Jain author Amitagati.
For this reason, multiple authors believe that the Vikram Samvat was not started by Vikramaditya, who might be a purely legendary king or the title adopted by a later king who renamed the era after himself. V. A. Smith and D. R. Bhandarkar believed that Chandragupta II adopted the title Vikramaditya, and changed the name of the era to "Vikrama Samvat". According to Rudolf Hoernlé, the king responsible for this change was Yashodharman: Hoernlé also believed that he conquered Kashmir, and is the same person as the "Harsha Vikramaditya" mentioned in Kalhana's Rajatarangini.
Earlier, some scholars believed that the Vikrama Samavat corresponded to the Azes era of the Indo-Scythian (Śaka) king King Azes. However, this was disputed by Robert Bracey following the discovery of an inscription of Vijayamitra, which is dated in two eras. The theory seems to be now thoroughly discredited by Falk and Bennett, who place the inception of the Azes era in 47–46 BCE.
The Vikrami era is an ancient calendar and has been historically used by Hindus and Sikhs. It is one of the several regional Hindu calendars that have been in use on the Indian subcontinent, and it is based on twelve synodical lunar months and 365 solar days. The lunar new year starts on the new moon in the month of Chaitra. This day, known as Chaitra Sukhladi, is a restricted holiday in India.
The Vikrami Samvat (Bikrami Samvat system) has been in use in the Indian subcontinent since ancient times, and remains in use by the Hindus in north, west and central India as well as Nepal. In south India, and some parts of east and west India such as Assam, West Bengal, and Gujarat, saka era has been widely used.
With the arrival of the Islamic rule era, the Hijri Islamic calendar became the official calendar of various Sultanates and the Mughal Empire. During the british colonial rule of the Indian subcontinent, the Gregorian calendar was adopted and it is commonly used in the urban areas of India.  The predominantly Muslim countries of Pakistan and Bangladesh use the Islamic calendar since 1947, but older texts variously included the Bikrami and Gregorian calendar systems. In 2003, the India-based Shiromani Gurdwara Parbandhak Committee of Sikhism adopted the Nanakshahi calendar, a move that continues to be debated. The Vikrami calendar is the official calendar of Nepal.
The calendar system
The Vikrami calendar is similar in conceptual design to the Gregorian calendar, but different from the Jewish 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 Vikrami and Jewish calendars maintain the integrity of the lunar month, but insert an extra full month by complex rules, every 4 years, to ensure that the festivals and crop-related rituals fall in the appropriate season. This Indian system of calendar keeping is one of the lunisolar calendar systems innovated in ancient human cultures. 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 Vikram Samvat has two alternative systems. It started in 56 BCE in the southern (purnimanta) and 57–56 BCE in the northern (amanta) systems of the Hindu calendar. The Shukla Paksha, when most festivals occur, coincides in both systems. The era is named after King Vikramaditya of India.
The lunisolar Vikram Samvat calendar is 56.7 years ahead of the solar Gregorian calendar. For example, the year 2075 VS began in 2018 CE and will end in 2019 CE.
The Rana rulers of Nepal made Vikram Samvat the official Hindu calendar in 1901 CE, which started as Samvat 1958. In Nepal, the new year begins with the first day of the month of Baishakh, which usually falls around 13–15 April in the Gregorian calendar. The first day of the new year is passionately celebrated in a historical carnival that takes place every year in Bhaktapur, called Bisket Jatra. From 2007 AD, Nepal Sambat is also recognized as the national calendar.
In India, the reformulated Saka Calendar is officially used (although not for computing the dates of the traditional festivals). In the Hindi version of the Preamble of the Constitution of India, however, the date of adoption of the constitution, 26 November 1949, is presented in Vikram Samvat (Margsheersh Shukla Saptami Samvat 2006). There have been calls for the Vikram Samvat to replace Saka as India's official calendar.
Divisions of a year
The classical Vikram Samvat uses lunar months and solar sidereal years. Because 12 months do not match a sidereal year exactly, correctional months (adhika māsa) are added or, occasionally, subtracted (kshaya masa). A lunar year consists of 12 months. A lunar month has two fortnights. The lunar days are called "tithis". Each month has 30 tithis, which may vary from 20 – 27 hours. During the waxing phases, tithis are called "shukla" or the bright phase — the auspicious fortnight, beginning with the day after the new moon called "Amavasya". Tithis for the waning phases are called "krishna" or the dark phase, which is regarded as the inauspicious fortnight, starting from the day after the full moon or "purnima".
- A Tithi (or lunar day) is defined as the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to 26 hours.
- A Paksa (also Pakṣa) or lunar fortnight consists of 15 tithis.
- A Māsa or lunar month (approximately 29.5 days) is divided into 2 Pakṣas: the one between new moon and full moon (waxing) is called gaura (bright) or Śukla Pakṣa; the one between full moon and new moon (waning) Kṛiṣhṇa (dark) or Vadhya Pakṣa
- A Ṛitu (or season) is two Māsa
- An Ayana is three Ṛitus
- A year is two Ayanas 
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