There are eight different opinions regarding when Job lived, which are noted in the Talmud (Hebrew Oral Torah, the text Christians disregarded and substituted for New Testament). There is even a ninth opinion that the story of Job is a parable, that he never lived at all (Talmud, Bava Basra 15a-b).
Maharal explains that the theme of suffering is universal and cannot be isolated to any particular period. That is why (as you noted) this book is quite unique, it does not say when or in what context the events took place.
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