Due date ?

@Learner (searching) (91 Points)

21 July 2015  
Mr X is having turnover 5000000 and income Rs 400000 and he wants to audit his books of accounts. as per income tax act, it is not required but he voluntarily wants to get audited. in this situation what will be the due date ? 31.07.2015 or 30.09.2015 ? he is individual and he is having retail business. please answer in detail and provide case law, if any. thank you