Zoroaster is said to have lived in the sixth century B.C. This has been a tradition, within Zoroastrianism itself, for around two thousand years. Up until recently in the West its accuracy was widely considered an established fact.
Western scholars came across the evidence for this tradition in the nineteenth century. Many immediately gave it a warm reception. After all, such a late dating was far better suited to the forward-looking, evolutionary spirit of the times than the much earlier one that had been favoured up until then, but which risked losing sight of Zoroaster in a suspiciously distant past. Also, as well as sounding eminently reasonable it seemed to have the pedigree of a genuine Zoroastrian tradition.