When did Darius attack Indus valley Region?

516 BC is the year mentioned by some.

http://www.historycentral.com/dates/600bc.html

http://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/Asia/India/_Texts/OXFHOI/1/3*.html

http://web.ics.purdue.edu/~rauhn/ancindia.