Bangabda Bangla calendar, also known as 'Bangla Sal', was promulgated by the Mughal emperor akbar in 1584 AD. The new calendar introduced on 10 or 11 March 1584 was initially known as Tarikh-e-Elahi. Though the new calendar was promulgated in the twenty-ninth year of Akbar's reign, it dates from his ascension to the throne on 5 November 1556.

The purpose of Tarikh-e-Elahi was to glorify Akbar's ascent to the throne as well as to give a new calender for the collection of revenue. Hitherto, the Mughal emperors had been using the Hijri calendar for the purposes of collecting revenue. However, as Abul Fazl explains in Akbar Namah, the use of the Hijri calendar was irksome to the peasantry because there was a difference of 11 or 12 days between the lunar and the solar years, with 31 lunar years being equal to 30 solar years. Revenue was collected according to the lunar year, whereas the harvest was dependent on the solar one. From the beginning of his reign, Akbar had felt the need of introducing a uniform, scientific, and workable system of calculating days and months through a reformed calendar. With this end in view, he commissioned Amir Fathullah Shirazi, a distinguished scientist and astronomer, to make the changes.

Accordingly, the first of muharram 963 AH was also made the starting point of 963 of Tarikh-e-Elahi. Since the month of Muharram 963 AH coincided with the month of Baishakh, the month of Baisakh was made the first month of the new era instead of the month of Chaitra which was the first month of the shakabd, then being used in Bengal.

During the four hundred odd years that have elapsed since the Tarikh-e-Elahi was promulgated, a difference of 14 years has arisen between the Hijri and Bangla calendars. The Islamic Hijri calendar is a lunar while the Bangla calendar is a solar one. The lunar year is 11 days shorter than the solar year. Hence the difference that has arisen between the Hijri calendar and the Bangla one: 2002 is 1408 of the Bangla year but 1422 of the Hijri year. The difference between the Bangla year and the Gregorian year, both of which are solar years, has remained the same. At the time of the introduction of the Tarikhe-e-Elahi, the difference between the Gregorian and Hijri years was 1556-963=593 years, and the difference in 2002 remains the same: 2002-1409=593 years.

During the reign of Akbar, each day of the month used to have a different name. As it was too cumbersome to count the 31 names of the days of the month, Akbar's grandson, shahjahan, brought it down to a weekly system in his fasli san (agricultural calendar). His seven days of the week are similar to the week in the western calendar, with the Bangla week also starting from Sunday.

1. Rabi for Sun (Sunday)

2. Som for Moon (Monday)

3. Mangal for Mars (Tuesday, or Tiwes Daeg, ' the day of Tiw, Mars, the god of war)

4. Budh for Mercury (Wednesday)

5. Brihaspati for Jupiter (Thursday)

6. Shukra for Venus (Friday)

7. Shani for Saturn (Saturday).

The names of the months of the year were also changed. The months of the year were initially known as Farwardin, Khordad, Teer, Murdad, Shahrivar, Aban, Azar, Dey, Bahman etc. It is not known why the months were given the names Baisakh, Jyaistha, etc, but it is presumed that the names were derived from the Shakabda which had been introduced in 78 AD to commemorate the reign of the Saka Dynasty. The names of the months, as derived from different stars, were as follows:

1. Baishakh from Vishakha (Librae)

2. Jyaistha from Jaistha (Scorpii)

3. Asadh from Asadha (Sagittarii)

4. Shravan from Shravana (Aquilae)

5. Bhadra from Bhadrapada (Pegasi)

6. Ashvin from Ashvini (Arietis)

7. Kartik from Krttika (Tauri)

8. Agrahayan from Agraihani (Aldebaran)

9. Paus from Pusya (Cancri)

10. Magh from Magha (Regulus)

11. Falgun from Falguni (Leonis)

12. Chaitra from Chitra (Virginis)

The length of a year in the Bangla calendar, as in the Gregorian calendar, is counted as 365 days. However, the actual time taken by the earth in its revolution around the sun is 365 days 5 hours 48 minutes and 47 seconds. To make up this discrepancy, the Gregorian calendar adds an extra day to the month of February every fourth year (except in century years not divisible by 400). The Bangla year did not take into account these extra hours. Bangla months too were of different lengths. In order to counter this discrepancy and make the Bangla calendar more precise, a committee to reform the Bangla calendar was set up on 17 February 1966 under the auspices of the bangla academy and under the guidance of muhammad shahidullah. Under the recommendations of the committee, the months from Baisakh to Bhadra were to be counted as of 31 days each, while the months from Asvin to Chaitra were to be considered as of 30 days, with Chaitra having 31 days every four years. [Syed Ashraf Ali]

See also panjika; seasons.