Surya Siddhanta
पृथक्स्थास् ते +अधिमासघ्नाः सूर्यमासविभाजिताः। लब्धाधिमासकैर् युक्ता दिनीकृत्य दिनान्विताः ॥
Write down the result separately; multiply it by the number of additive months (in a yuga) and divide the product by the number of solar months (in a yuga); the quotient, (without the remainder,) will be the elapsed additive months. Add the quotient (without the remainder) to the said result, reduce the sum to days (by multiplying it by thirty) and increase it by the number of (lunar) days (passed of the present lunar month).
english translation
pRthaksthAs te +adhimAsaghnAH sUryamAsavibhAjitAH। labdhAdhimAsakair yuktA dinIkRtya dinAnvitAH ॥
hk transliteration by Sanscript