Kali Yuga is said to start at around 3000 BCE because Aryabhata said he is living 3600 years into Kali Yuga.
But how do we know when Aryabhata lived?
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In his book Aryabhatiya, he mentions the following:
"When the three yugas Satya yuga, Treta yuga and Dwapara yuga) have elapsed and 60 x 60 (3,600) years of Kali yuga have already passed, I am now 23 years old".