How to calculate interet on late payment of tds

Please explain me how to calculate interet on late payment of TDS.

Say, we deducted TDS on 31.05.2013 and paid TDS on 09.06.2013. In this situation, for how many months we should calculate interest. For 2 months or 1 month???  TDSCPC is calculating interest for two months in this situation. Is that correct? We paid interest for 1 month, as there is delay of only 2 days. We received Notice from TDSCPC asking us to pay interest for 2 months. 

When TDS returns were processed by NSDL, they accepted interest for 1 month. We did not receive any notice.

I request you all to provide your advice on this issue.

Regards

M. Suresh