Surya Siddhanta
स्फुटतिथ्यवसाने तु मध्यग्रहणम् आदिशेत् । स्थित्यर्धनाडिकाहीने ग्रासो मोक्षस् तु सम्युते॥
At the end of the true lunar day (i. e. at the time of the full moon) the middle of the eclipse takes place; this time diminished by the exact first Sthityardha leaves the time of the beginning, and increased by the latter exact Sthityardha gives the time of the end.
english translation
sphuTatithyavasAne tu madhyagrahaNam Adizet । sthityardhanADikAhIne grAso mokSas tu samyute॥
hk transliteration by Sanscript