Surya Siddhanta
तद्वद् एव विमर्दार्धनाडिकाहीनसम्युते। निमीलनोन्मीलनाख्ये भवेताम् सकलग्रहे॥
In the same manner, the time of the middle of a total eclipse diminished and increased (separately) by the exact first and second mardárdhas gives the times of the beginning and end of the total darkness (respectively).
english translation
tadvad eva vimardArdhanADikAhInasamyute। nimIlanonmIlanAkhye bhavetAm sakalagrahe॥
hk transliteration by Sanscript