Surya Siddhanta
अर्कोनचन्द्रलिप्ताभ्यस् तिथयो भोगभाजिताः। गता गम्याश् च षष्टिघ्न्यो नाड्यो भुक्त्यन्तरोद्धृताः॥
ake the place of the Sun from that of the Moon (found at a given time); divide the remainder, reduced to minutes, by the Bhoga (of a tithi or 720′; the quotient is the number of the elapsed tithis or lunar days.) (The remainder is the gata of the present tithi, and the Bhoga of a tithi diminished by the gata is the gamya of the present tithi.) The gata and gamya of the present tithi, multiplied by 60 and divided by the difference between the diurnal motions (of the Sun and Moon) become the numbers of the past and to be past ghaṭikás (respectively of the present tithi at the given time).
english translation
arkonacandraliptAbhyas tithayo bhogabhAjitAH। gatA gamyAz ca SaSTighnyo nADyo bhuktyantaroddhRtAH॥
hk transliteration by Sanscript