Srimad Bhagavatam Canto 12, Chapter 02, Text 33

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.
Srimad Bhagavatam Canto 12, Chapter 02, Text 32
Srimad Bhagavatam Canto 12, Chapter 02, Text 34