Gta service st deposit due date

You answer is within your question!!! In case of reverse charge, Rule 7 of Point of Taxation Rules, 2011 is applicable whereby date of payment is the point of taxation. Since you paid freight in september, the due date as per Rule 6(1) of STR, 1994 is 6th October (E-Payment). So, no need to pay interest. If the payment is not made within 6 months of date of invoice, then Rule 7 of POTR is not applicable and interest payable. 

Dear Sir, This was exactly my explanation to the Service Tax Inspector of my Range, but he says that I need to deposit Interest. What should tell him?

You may contact the Range Superintendent and inform him. As per law, you are not liable to pay interest. The inspector may verbally ask you to pay penalty also in addition to the interest, but that's wrong. You may write a letter to the suprentendent explaining your legal position, without waiting for any notice.