Surya Siddhanta
प्राक्पश्चान् नतनाडीभिस् तस्माल् लङ्कोदयासुभिः। भानौ क्षयधने कृत्वा मध्यलग्नम् तदा भवेत् ॥
(Of the given place of the Horoscope and that of the Sun), find the Bhogya time in Práṇas, of the less and the Bhukta time, in Práṇas of the greater, add together these times and the rising periods of the intermediate signs (between those which are occupied by the Sun and the Horoscope); and you will find the time (from sun-rise at the end of which the given place of the Horoscope is just rising in the eastern horizon).
english translation
prAkpazcAn natanADIbhis tasmAl laGkodayAsubhiH। bhAnau kSayadhane kRtvA madhyalagnam tadA bhavet ॥
hk transliteration by Sanscript