Agni Purana (Jyotisha Selections) · by Vyasa
Chapter 1 · Muhurta: Marriage, Home, Farming
ज्योतिःशास्त्रम्
सषष्टिकं प्रदेयं तत्षष्ठ्याधिक्ये च तत्त्यजेत् ।नक्षत्रं तिथिमिश्रं स्याच्चतुर्भिर्गुणिता तिथिः ॥ २१ ॥
In the full Agni Purana this is chapter 122, verse 21.
add sixty and then take it away. If the result is over sixty, drop sixty. The star is worked from the tithi (moon day). Multiply the moon day by 4.
Simple meaning: If the minus is too big, borrow sixty. Above sixty, drop sixty. The star is found from the moon day.
रविचन्द्रौ समौ कृत्वा योगो भवति निश्चलः ।
Adding the Sun and Moon gives the almanac's daily combination. The moon day, worked by seven, gives its half.
तिथिश्च द्विगुणैकोना कृताङ्गैः करणं निशि ।
A sum on the moon day gives the half for the night. Fixed halves fall at the month's end and start.
षष्ट्यब्दानां प्रवक्ष्यामि शुभाशुभमतः शृणु ।
In Prabhava there are sacrifice rites. In Vibhava people are happy.