According to Shamsuzzaman Khan, and Nitish Sengupta, the origin of the Bengali calendar is unclear.
Some historians attribute the Bengali calendar to the 7th century Hindu king Shashanka. The term Bangabda (Bangla year) is found too in two Shiva temples many centuries older than Akbar era, suggesting that a Bengali calendar existed long before Akbar's time.
Hindus developed a calendar system in ancient times. Jyotisha, one of the six ancient Vedangas, was the Vedic era field of tracking and predicting the movements of astronomical bodies in order to keep time. The ancient Indian culture developed a sophisticated time keeping methodology and calendars for Vedic rituals.
The Hindu Vikrami calendar is named after king Vikramaditya and starts in 57 BCE. In rural Bengali communities of India, the Bengali calendar is credited to "Bikromaditto", like many other parts of India and Nepal. However, unlike these regions where it starts in 57 BCE, the Bengali calendar starts from 593 CE suggesting that the starting reference year was adjusted at some point.
Various dynasties whose territories extended into Bengal, prior to the 13th-century, used the Vikrami calendar. For example, Buddhist texts and inscriptions created in the Pala Empire era mention "Vikrama" and the months such as Ashvin, a system found in Sanskrit texts elsewhere in ancient and medieval Indian subcontinent.
Hindu scholars attempted to keep time by observing and calculating the cycles of sun (Surya), moon and the planets. These calculations about the sun appears in various Sanskrit astronomical texts in Sanskrit, such as the 5th century Aryabhatiya by Aryabhata, the 6th century Romaka by Latadeva and Panca Siddhantika by Varahamihira, the 7th century Khandakhadyaka by Brahmagupta and the 8th century Sisyadhivrddida by Lalla. These texts present Surya and various planets and estimate the characteristics of the respective planetary motion. Other texts such as Surya Siddhanta dated to have been complete sometime between the 5th century and 10th century.
The current Bengali calendar in use by Bengali people in the Indian states such as West Bengal, Tripura, Assam and Jharkhand is based on the Sanskrit text Surya Siddhanta. It retains the historic Sanskrit names of the months, with the first month as Baishakh. Their calendar remains tied to the Hindu calendar system and is used to set the various Bengali Hindu festivals.
Influence on Islamic Calendar
Another theory is that the calendar was first developed by Alauddin Husain Shah (reign 1494–1519), a Hussain Shahi sultan of Bengal by combining the lunar Islamic calendar (Hijri) with the solar calendar, prevalent in Bengal. Yet another theory states that the Sasanka calendar was adopted by Alauddin Husain Shah when he witnessed the difficulty with collecting land revenue by the Hijra calendar.
During the Mughal rule, land taxes were collected from Bengali people according to the Islamic Hijri calendar. This calendar was a lunar calendar, and its new year did not coincide with the solar agricultural cycles. According to some sources, the current Bengali calendar owes its origin in Bengal to the rule of Mughal Emperor Akbar who adopted it to time the tax year to the harvest. The Bangla year was therewith called Bangabda. Akbar asked the royal astronomer Fathullah Shirazi to create a new calendar by combining the lunar Islamic calendar and solar Hindu calendar already in use, and this was known as Fasholi shan (harvest calendar). According to some historians, this started the Bengali calendar. According to Shamsuzzaman Khan, it could be Nawab Murshid Quli Khan, a Mughal governor, who first used the tradition of Punyaho as "a day for ceremonial land tax collection", and used Akbar's fiscal policy to start the Bangla calendar.
It is unclear whether it was adopted by Hussain Shah or Akbar. The tradition to use the Bengali calendar may have been started by Hussain Shah before Akbar. According to Amartya Sen, Akbar's official calendar "Tarikh-ilahi" with the zero year of 1556 CE was a blend of pre-existing Hindu and Islamic calendars. It was not used much in India outside of Akbar's Mughal court, and after his death the calendar he launched was abandoned. However, adds Sen, there are traces of the "Tarikh-ilahi" that survive in the Bengali calendar. Regardless of who adopted the Bengali calendar and the new year, states Sengupta, it helped collect land taxes after the spring harvest based on traditional Bengali calendar, because the Islamic Hijri calendar created administrative difficulties in setting the collection date.
Shamsuzzaman states, "it is called Bangla san or saal, which are Arabic and Parsee words respectively, suggests that it was introduced by a Muslim king or sultan." In contrast, according to Sengupta, its traditional name is Bangabda. In the era of the Akbar, the calendar was called as Tarikh-e-Elahi (তারিখ-ই ইলাহি). In the "Tarikh-e-Elahi" version of the calendar, each day of the month had a separate name, and the months had different names from what they have now. According to Banglapedia, Akbar's grandson Shah Jahan reformed the calendar to use a seven-day week that begins on Sunday, and the names of the months were changed at an unknown time to match the month names of the existing Saka calendar. This calendar is the foundation of the calendar that has been in use by the people of Bangladesh.