Reference : History of Astronomy in India. Indian National Science Academy.1985. Chapter 9. Indian Calendar from Post-Vedic period to AD 1900. Page 291: S.K. Chatterjee and A.K. Chakravarthy.
Calendars change from time to time and country to country, making it difficult to translate between calendars and understand “when” something actually happened either from a historical perspective or from an astronomical perspective.
Aryabhatta was the first astronomer in the world to conceive the brilliant idea of counting in a continuous manner the days without reference to months and years and designating each day by a number. This was called Ahargana. More than a thousand years later, a Julian Day Number was started by a French Scholar Joseph Scaliger in CE (AD) 1582.
Aryabhatta used as his zero day, the beginning of the Yuga. (Mahayuga, in our terms.) Later astronomers found that the number of days was too big for ease. So they took the starting day as the start of the Kaliyuga, which commenced from the midnight of 17-18 February 3102 BCE (BC). Ahargana day is assigned on the basis of the number of mean solar days that have elapsed at midnight time At Ujjain, from the beginning of the Kaliyuga.
The Julian Day Numbers use as the zero time, Greenwhich mean noon of 1st January, 4713 BCE (BC).
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