Surya Siddhanta
तत्कार्मुकम् उदक्क्रान्तौ *धनशनी पृथक्स्थिते ।(C धनहानी) स्वाहोरात्रचतुर्भागे दिनरात्रिदले स्मृते ॥
Add and subtract the ascensional difference to and from the fourth part of the length of the day and night of the planet (as found in s'loka 59) separately, the results will be lengths of the half day and half night respectively of the planet when its declination is north.
english translation
tatkArmukam udakkrAntau *dhanazanI pRthaksthite ।(C dhanahAnI) svAhorAtracaturbhAge dinarAtridale smRte ॥
hk transliteration by Sanscript