Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 2 · Signs, Planets, Birth
जातकम् १
लग्नायुरंशास्त्रिगुणा दिग्भक्ता स्युः समास्ततः ।शेषेऽर्कादिगुणे भक्ते दिग्भर्मासादिकं भवेत् ॥ १३४ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 55, verse 134.
Multiply the lagna (rising sign)'s lifespan parts by 3 and divide by 10: this gives years. Multiply the remainder by 12 and divide by 10: this gives months and the rest.
Simple meaning: The rising sign's parts are turned into years and months.
सबलेंगेभतुल्याब्दैर्युतमायुर्भवेत्स्फुटम् ।
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.