Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
पपीयाद्गणयेद्भानि रोहिण्यादिचतुर्विधम् ।स्थिरभेष्वर्कसंक्रांतिर्ज्ञेया विष्णुपदाह्वया ॥ २६२ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 262.
Count the nakshatra (star) from Rohini, in four kinds. The Sun's sankranti (the Sun's entry into a sign) into fixed signs is called Vishnupada.
Simple meaning: Count the stars from Rohini in four kinds. The Sun's entry into a fixed sign is Vishnupada.
षडशीतिमुखा ज्ञेया द्विस्वभावेषु राशिषु ।
The Sun's entry into Libra and Aries is the equinox.
याम्यायने स्थिरे त्वाद्याः पराः सौम्येन्दुमूर्तिभैः ।
Holy hours fall before or after the Sun's entry, by its course and sign. Sixteen ghatikas are holy.
संध्या त्रिनाडी प्रमितार्कबिंबोर्द्धोदयास्ततः ।
Twilight lasts three ghatikas. An entry in twilight moves its holy time to the day before or after.