Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
कार्तिके शुक्लनवमी त्वादिः कृतयुगस्य च ।त्रेतादिर्माधवे शुक्ले तृतीया पुण्यसंज्ञिता ॥ १४७ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 147.
The 9th of the bright fortnight in Kartika is the first day of the Krita age. The 3rd of the bright fortnight in Madhava (Vaishakha) is the first day of the Treta age. It is called holy.
Simple meaning: The Krita age begins on the bright 9th of Kartika, and the Treta age on the bright 3rd of Vaishakha.
कृष्णा पंचदशी माघे द्वापरादिमुदीरिता ।
The Dvapara age begins on the dark new moon of Magha, and a kalpa on the dark 13th of Nabhasya.
द्वादश्यूर्जे शुक्लपक्षे नवम्यच्छेश्वयुज्यपि ।
More holy days: bright 12th in Urja and bright 9th in Ashvina. The bright 3rd of Chaitra and Bhadrapada is called Shukla.
एकादशी सिता पौषे ह्याषाढेर्देशमीसिता ।
More holy days: the bright 11th of Pausha, the 10th of Ashadha, the 7th of Magha and the dark 8th of Nabhasya.