Tamil calendar

It is used in contemporary times for cultural, religious and agricultural events,[3] with the Gregorian calendar largely used for official purposes both within and outside India.

This is related to 5 12-year revolutions of Jupiter around the Sun and one that adds up to 60 years and the orbit of Nakshatras (stars) as described in the Surya Siddhanta.

Nakkeerar, Sangam period author of the Neṭunalvāṭai, wrote in the third century CE that the Sun travels each year from Mesha/Chittirai in mid-April through 11 successive signs of the zodiac.

[4] Kūdalūr Kiḻar in the third century CE refers to Mesha Rāsi/Chittirai i.e. mid-April as the commencement of the year in the Puṟanāṉūṟu.

[7] The 5th century Silappadhigaaram mentions the 12 rāsigal or zodiac signs that correspond to the Tamil months starting with Mesha/Chittirai in mid-April.

[12] This also coincides with the traditional new year in Burma, Cambodia, Laos, Sri Lanka, Bangladesh, Nepal, and Thailand.

The days of week (Kiḻamai) in the Tamil calendar relate to the celestial bodies in the solar system: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, in that order.

Its earliest reference is to be found in Surya Siddhanta, which Varahamihirar (550 CE) believed to be the most accurate of the then current theories of astronomy.

[14][15][16] The Surya Siddhanta and other Indian classical texts on astronomy had some influence on the Chinese calendar[17] although it merits attention that the sexagenary cycle in China is itself very old.