What is the earliest record of Mahabharata

What is the earliest record of the Kali Santarana Upanishad, source of the Hare Krishna mantra?

"It refers to the conversation at the end of the Dvapara Yuga, which only makes sense if the reader is believed to live long after the beginning of the Kali Yuga etc."

satyayuga-> tretayuga-> dwaparayuga-> kaliyuga-> satyayuga-> tretayuga-> dwaparayuga-> kaliyuga-> satyayuga-> tretayuga-> dwaparayuga-> kaliyuga -> ...... (continues)

After Kali-Yuga comes Satya-Yuga again and the cycle of Satyayuga to Kaliyuga is repeated several times until it becomes 1000 Yugas, which represent a day of Lord Brahma, as stated in Bhagavad-Gita 8.17

In Bhagavad-Gita 8.17 it says "Sahasra-Yuga-Paryantam Ahar Yad Brahmano Viduh ...".

So this Kali-Yuga is neither the first Kali-Yuga of creation nor the last. This Upanishad, which refers to the end of the Dwapara Yuga, does not necessarily have to mean that this covering only took place at the end of this Dwapar-Yuga. However, the basic knowledge of how to cross the Kali-Yuga remains essentially the same.

So do not think that "since this Upanishad refers to the end of Dwaparyug, it should only arise at the end of this Dwapar-yug and not exist before". The conversation took place at the end of a dwapar-yuga in the eternal timescale. However, the essential knowledge in the conversation remains the same throughout eternal time. So the conversation is always informative. The Upanishad is eternal. It's not that it was born just before that Kali Yuga.