Śrīmad-Bhāgavatam · Text 12.2.33
Who is speaking and story frame
यस्मिन् कृष्णो दिवं यातस्तस्मिन्नेव तदाहनि ।
प्रतिपन्नं कलियुगमिति प्राहु: पुराविद: ॥ ३३ ॥
yasmin kṛṣṇo divaṁ yātas
tasminn eva tadāhani
pratipannaṁ kali-yugam
iti prāhuḥ purā-vidaḥ
Those who scientifically understand the past declare that on the very day that Lord Śrī Kṛṣṇa departed for the spiritual world, the influence of the Age of Kali began.
Read the purport on Vedabase.ioFrom Canto 10, chapter 14 onward, the translation and purports were completed by Śrīla Prabhupāda's disciples after his departure, and published by the Bhaktivedanta Book Trust.