El-Nashar was detained July 14 in Cairo after Britain notified Egyptian authorities that it suspected he may have had links to some of the attackers.
Seventeen days after the day El-Nashar was held would be the last day of July.
July 31 is 17 days after the day he was caught so it is correct but I think the AI thinks it was saying this was the day he was detained.
( z1 day :mod ( z2 last ) :part-of ( z3 date-entity :month 7 ) :domain ( z4 after :op ( z5 hold :ARG1 ( z6 event :name ( z7 name :op "El-Nashar" ) ) ) :quant ( z8 temporal-quantity :quant 17 :unit ( z9 day ) ) ) )