Brihat Parashara Hora Shastra
एवं महादशारम्भो भवेदंशाद्यथा क्रमात् । गणयेन्नवपर्यन्तं तत्तदायुः प्रकीर्तितम् ॥ ९१॥
The result will indicate the expired period of the Dasha in years, months etc. By deducting it from the total number of years allotted, we get the balance of Dasha at birth. The Dasha should be taken, as commencing from that Rāśi.
english translation
evaM mahAdazArambho bhavedaMzAdyathA kramAt । gaNayennavaparyantaM tattadAyuH prakIrtitam ॥ 91॥
hk transliteration by Sanscript