Brihat Parashara Hora Shastra
अस्तकाले च या सन्ध्या सुधाख्या दशनाडिका । पूर्णमुग्धघटीमाने द्विगुणे तिथिभिर्भजेत् ॥ ४७॥
If the birth is in Purna, or Mugdha, its past Ghatikas should be multiplied by 2 and the product should be divided by 15.
english translation
astakAle ca yA sandhyA sudhAkhyA dazanADikA । pUrNamugdhaghaTImAne dviguNe tithibhirbhajet ॥ 47॥
hk transliteration by Sanscript