SB 12.2.33

yasmin kṛṣṇo divaṁ yātas
 tasminn eva tadāhani
pratipannaṁ kali-yugam
 iti prāhuḥ purā-vidaḥ
Word for word: 
yasmin — on which; kṛṣṇaḥ — Lord Śrī Kṛṣṇa; divam — to the spiritual world; yātaḥ — gone; tasmin — on that; eva — same; tadā — then; ahani — day; pratipannam — obtained; kali-yugam — the Age of Kali; iti — thus; prāhuḥ — they say; purā — of the past; vidaḥ — the experts.
Translation: 
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.
Purport: 

Although technically Kali-yuga was to begin during the time of Lord Kṛṣṇa’s presence on earth, this fallen age had to wait meekly for the departure of the Supreme Personality of Godhead.