Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · Signs, Planets, Birth
जातकम् १
सबलेंगेभतुल्याब्दैर्युतमायुर्भवेत्स्फुटम् ।अंशद्विघ्नमक्षांशं मासाः खत्र्यादिसंगुणात् ॥ १३५ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 55, verse 135.
If the lagna (rising sign) is strong, add as many years as its signs. Then the life is exact. Double the degrees and divide by 5: months. Multiply the remainder by 30 and so on.
Simple meaning: A strong rising sign adds years equal to its signs, and months from its degrees.
शेषा दिनादिकं योज्यं नैतत्पिंडनिसर्गयोः ।
The strongest among the rising sign, Sun and Moon has its period first.
ततः केंद्रादिगानां तु द्वित्र्यादौ सबलस्य च ।
Planet periods run in order of house, strength and years.
षड्वर्गार्द्धस्य त्रिंशस्य त्रिकोणगश्च स्मरगः ।
Planets share the sub-periods by their house from the period lord.