The Calendar followed by Government of India along with Gregorian Calendar is Indian National Calendar or Saka Calendar.
As an ancient civilisation India had several similar Calendars followed by various cultures in India. After independence, the first Prime Minister of India - Pandit Jawaharlal Nehru established a Committee to develop a Uniform Calendar by doing a detailed study of different calendars prevalent in different parts of the country. This led to the development of Indian National Calendar of Saka Calendar.
The Saka Calendar counts years with Saka era, for which the Epoch begins in the year AD -78. The Saka era is variedly called as Shalivahana Saka era, the Greater Era and Major Era.
The Saka Calendar is a Solar Calendar which tracks the tropical year and starts on the day after the March Equinox day. The tropical year is also called as Sayana Year in India. The months in Saka Calendar has fixed number of days in each month : The first six months with 31 days and next six months with 30 days(the first month with 30 days in normal years and with 31 days in leap years). The names of the months were given from old lunisolar calendars that were prevalent in India.
|Saka Month Number||Saka Month Name||Number of days||Gregorian Month Start Date|
|1||Chaitra||30 during Normal Year 31 during Leap Year||22 March during Normal Year 21 March during Leap Year|
The official usage of the Indian National Calendar or the Saka Calendar started on Chaitra 1, 1879, Saka Era - the equivalent Gregorian date being March 22, 1957.
The Saka Calendar is not based on the Surya Siddhanta as it tracks the tropical year or Sayana Year and not the Nirayana Year.