‘So this Daniel prospered in the reign of Darius, even in the reign of Cyrus the Persian.'
Here the writer first refers to the king under whom Daniel prospered, and then to his overlord, Cyrus the Persian. This dating in the name of two contemporary kings is well testified to in inscriptions and records around that time. (Whether they are contemporary or successive kings cannot be determined from the text, which is neutral in this regard, and it would be dishonest to suggest otherwise on either side).