Ahargana is the number of days that elapse from a certain epoch up to a given date. In a solarcalendar, ahargana can be obtained simply by counting the days. However, the problem arises when dates are given using a luni-solar calendar, in which after a regular period, an intercalary month is added to match it with the solar calendar. Ahargana calculation essentially started with the Siddhanta texts (4th century CE). In Panchasiddhantika (535 CE), of Varahamihira, the weekday is obtained from ahargana.
Paulisa Siddhanta and Romaka-Siddhanta use ahargana to determine mean positions of the Sun and the Moon. Surya Siddhanta gives mahayuga system to calculate ahargana for finding the mean position of the planets too. The mahayuga, a period of 4, 32, 000 years, is divided into four yugas (ages) – Krita, Treta, Dwapara and Kali. All the heavenly bodies go through an integral number of revolutions in this period, and are assumed to be in conjunction at the first point of the zodiac in the beginning, and at the end of this long period, they are supposed to be in conjunction again.
If the number of civil days elapsed from the commencement of the epoch up to any point of time is found, then the mean longitude of any planet can be obtained by dividing the elapsed days by the time-period of the planet. The quotient thus obtained will denote the number of complete revolutions the planet performed, and the remainder, expressed in degrees, yields the mean longitude. As Siddhanta texts use mahayuga system, their calculations involve very large numbers. To simplify them, astronomical Tantra texts, like Sishyadhivriddhida Tantra composed by Lalla (768 CE) were written which determine ahargana from the beginning of kali yuga (3102 BCE). Later Karana texts were also prepared, in which any convenient epoch is selected.