Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
तत्तन्मासैर्द्वादशभिस्तत्तदष्टौ भवेत्ततः ।गुरुमध्यमचारेण षष्ट्यब्दाः प्रभवादयः ॥ ११४ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 114.
Twelve months of each kind make a year of that kind. By the mean motion of Jupiter come the sixty years, Prabhava and the rest.
Simple meaning: Twelve months of any kind make a year. Jupiter's average movement gives the sixty named years.
प्रभवो विभवः शुक्लः प्रमोदोऽथ प्रजापतिः ।
These are the first ten names of the sixty-year cycle.
ईश्वरो बहुधान्यश्च प्रमाथी विक्रमो वृषः ।
These are the next ten names of the sixty-year cycle.
सर्वजित्सर्वधारी च विरोधी विकृतः खरः ।
These are the next ten names of the sixty-year cycle.