Surya Siddhanta
वैशाखादिषु कृष्णे च योगः पञ्चदशे तिथौ। कार्त्तिकादीनि वर्षाणि गुरोर् अस्तोदयात् तथा॥
Just as in Vaiçäkha etc., the month gets its name from the star corresponding to the tithi during the full moon, similarly, the year gets its name from the star corresponding to the lunar fifteenth day (kåishëacaturdaçi) during the dark half month when it is the time of Jupiter’s setting and rising.
english translation