The Zoroastrian scriptures are written in Avestan, an extremely close relative of Vedic Sanskrit; this dates it to the late Bronze Age, somewhere around 1000 BC

The Zoroastrian scriptures are written in Avestan, an extremely close relative of Vedic Sanskrit; this dates it to the late Bronze Age, somewhere around 1000 BC, closer to King David than Isaiah. The date 600 BC is Cyrus' period; later Persians appear to have confused the age of Cyrus, when Zoroastrianism entered the world stage, with that of Zoroaster. On a personal note: speaking as a Catholic and starting from those premises, I don't see any religious reason to worry about Zoroastrianism's influence on Judaism (or whatever it should be called at that time). God works slowly, and Zoroaster was a holy man living in the age of the prophets; there's no reason he couldn't have received an accurate partial revelation, rather like what Moses got. Judging from Biblical evidence, early Judaism (the Ritualistic Ten Commandments, diviners and soothsayers, the enthusiastic old narrative interleaved with the pessimistic new narrative in the Books of Samuel) would be very unsettling if we saw it today; while post-Zoroastrian-contact Judaism, with angels, devils, and a single supreme God, would be something theologically recognizable and familiar. (Now I'm wondering about the extent to which magic and astrology crossed over into Judaism from Zoroastrianism. Both practices arise naturally from Zoroastrian assumptions, with Ahura Mazda and Angra Mainyu as approximately peers and humanity as Ahura Mazda's ally, but not from later-Jewish ones...)