According to the Oxford Dictionary of Quotations, Einstein wrote in a letter to his friend Michelangelo Besso, dated 21 March 1955: "The distinction between past, present and future is only an illusion, however persistent".
According to Wikipedia however, Michele Angelo Besso died on 15 March 1955, and it was in a letter of condolence to his family that Einstein wrote "... the distinction between past, present and future is only a stubbornly persistent illusion." This is virtually the same (allowing for discrepancies in translation) as the wording quoted on Quora.com by one Tom McFarlane, who cites as his source The Expanded Quotable Einstein, edited by Alice Calaprice (Princeton University Press, 2000). He adds, "The erroneous quote in the original question ... is actually inconsistent with what Einstein thought. For Einstein, reality is what is true, not what is illusory."
The "erroneous quote" that Mr. McFarlane refers to is identical with the wording in the Macc QL question. It appears to be sourced from the unreliable Brainy Quote, which never seems to cite its sources and appears to have a somewhat cavalier attitude towards accuracy. Or, as Einstein might (or might not) have said, a regrettably poor grasp on reality.
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