delay period how to calucalate

   example  due date for service tax january 08, amount 10,00,000/-
  but we paid 10,00,000/- on  march 08- intrest not paid  .Service tax officials found on 01.06.08 not paid intreest for above amount- 

my doubt is  intrest payable for delayed period of  2 months or  as on 01.06.08 is  payable.pls help in these forum friends.