Original: The authenticity of an earlier anecdote of a windmill involving the second caliph Umar ( AD 634 -- 644 ) is questioned on the grounds that it appears in a 10th-century document .
Simplified: The authenticity of an earlier anecdote of a windmill involving the second caliph Umar ( 634-644 AD ) is questioned on the grounds of being a 10th century amendment .