There are many views on the timeline for Zoroaster's life. Greek sources placed him as early as 6000 BC. The traditional Zoroastrian date for Zarathushtra's birth and ministry is around 600 B.C. This is derived from a Greek source that places him "300 years before Alexander" which would give that date; other rationales for the 600 BC date identify the King Vishtaspa of Zarathushtra's Gathas with the father of the Persian King Darius, who lived around that time. According to the Zend Avesta, the sacred book of Zoroastrianism, Zoroaster was born in Azerbaijan, in northern Persia.