Surya Siddhanta
सावनो द्युगणः सूर्याद् दिनमासाब्दपास् ततः। सप्तभिः क्षयितः शेषः सूर्याद्यो वासरेश्वरः ॥
From the number of these elapsed days, the Rulers of the present day month and year can be known (by reckoning the order of them) from the Sun. Divide the number of elapsed terrestrial days by 7, and reckoning the remainder from the sun-day, the Ruler of the present day will be found.
english translation
sAvano dyugaNaH sUryAd dinamAsAbdapAs tataH। saptabhiH kSayitaH zeSaH sUryAdyo vAsarezvaraH ॥
hk transliteration by Sanscript