Reverse charge mechanism

I pay service tax on GTA services. Sometimes I pay local transporters in cash at factory. Now Iwant to know what is the treatment of service date with interest on late payment. Suppose I paid local transporter a sum of 20000 on 1.10.2015 and I passed the entry as

                                             carriage charges Dr.   20000

                                                 To cash at factory      20000

Now what is the service tax amount to b e paid and what will be the due date for interest calculation? Does interest on late payment calculate on monthly basis or day-to-day basis??

please answer me as per latest ammendments.