How is it possible ?
As you said He got registered on 22.08.2012. For this year till the time there are 2 returns available.
1st - Covers the period of April - June 12 , due date was 25.11.2012 , was made available on 15.10.2012
He is not required to file the return for this period.
2nd - Covers the period of July - Sept 12 , due date is 30.04.13 , was made available on 26.03.2013
You have to file NIL return if there is no transaction.
Now For December 2012 , if your client is Company then your due date is 05/01/2013 for Decemeber month payment and if he is other than Company then 05/01/2013 for october to december 2012.
Same for January : if company then 05/02/2013 if others 31/03/2013.
Now if you made delay in Interest than you have to pay int. @ 18% pa. Here you have to pay interest based on day basis. ( Also if your turnover for the period April to March 13 is less than 60 lakhs and if your client is other than company then interest rate is 13% - in case of new assessee) In case of existing assessee if the turnover of April - March 12 is less than 60 lakhs then also 13% pa int. is to be payable on delayed payment.