SB 12.2.33
yasmin krsno divam yatas
tasminn eva tadahani
pratipannam kali-yugam
iti prahuh pura-vidah
Translation:
Those who scientifically understand the past declare that on the very day that Lord Sri Krsna 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 Krsna’s presence on earth, this fallen age had to wait meekly for the departure of the Supreme Personality of Godhead.