* It's part of the Mahabharata: The Bhagavad Gita is a part of the epic Mahabharata, which is generally believed to have been composed around the 4th century BCE.
* Internal evidence: The text itself contains clues about its time period. For example, it mentions chariot warfare, which was prevalent in ancient India.
* Scholarly debate: There's ongoing debate among scholars about the exact date of composition. Some argue for an earlier date, while others favor a later one.
While the exact date of the Bhagavad Gita's composition is uncertain, it's generally considered to be an ancient text, likely dating back to the pre-Christian era.