Indian Astronomy Calendars

The divisions of the year were on the basis of religious rites and seasons (Rtu). The duration from mid March—Mid May was taken to be spring (vasanta), mid May—mid July: summer ("grishma"), mid July—mid September: rains (varsha), mid September—mid November: autumn, mid November—mid January: winter, mid January—mid March: dew (śiśira).

In the Vedānga Jyotiṣa, the year begins with the winter solstice. Hindu calendars have several eras:

• The Hindu calendar, counting from the start of the Kali Yuga, has its epoch on 18 February 3102 BC Julian (23 January 3102 BCE Gregorian).
• The Vikrama Samvat calendar, introduced about the 12th century, counts from 56–57 BCE.
• The "Saka Era", used in some Hindu calendars and in the Indian national calendar, has its epoch near the vernal equinox of year 78.
• The Saptarshi calendar traditionally has its epoch at 3076 BCE.

J.A.B. van Buitenen (2008) reports on the calendars in India:

The oldest system, in many respects the basis of the classical one, is known from texts of about 1000 BC. It divides an approximate solar year of 360 days into 12 lunar months of 27 (according to the early Vedic text Taittirīya Saṃhitā 4.4.10.1–3) or 28 (according to the Atharvaveda, the fourth of the Vedas, 19.7.1.) days. The resulting discrepancy was resolved by the intercalation of a leap month every 60 months. Time was reckoned by the position marked off in constellations on the ecliptic in which the Moon rises daily in the course of one lunation (the period from New Moon to New Moon) and the Sun rises monthly in the course of one year. These constellations (nakṣatra) each measure an arc of 13° 20′ of the ecliptic circle. The positions of the Moon were directly observable, and those of the Sun inferred from the Moon's position at Full Moon, when the Sun is on the opposite side of the Moon. The position of the Sun at midnight was calculated from the nakṣatra that culminated on the meridian at that time, the Sun then being in opposition to that nakṣatra.

Source: Wiki