Answer: This is actually a great question and kind of depends on who you ask. Historians will say that the Upanishads were written between 700 BC to 500 BC but if your a Hindu you might believe that these texts weren’t written by mere mortals like you and me but actually always existed and are pieces of Divine philosophy. I won’t pretend to know the ins and outs of Hinduism so I won’t go any further, I hope this helped.