Hi Rajesh/Ajay,
I have just run the interest calculation FINT and I have no problem with days. I posted one invoice dated 2013/10/30 and payment term 0001 i.e. immediate pay. Executed the calculation FINT and mention "Interest calculation to" 2013/10/31 in general selection. System calculated the interest for one day i.e. 2013/10/31 which is perfect. Then I changed this date to calculated up to 2013/11/01. System provided calculation for two day i.e. from 2013/10/31 to 2013/11/01 which is again perfect. Can you please check what is the posting date in your invoice... I kept document date and posting date same. In my Item Interest Calculation setup I have selected posting date as reference date. See if you get the same result if document and posting dates are kept same.
Thanks
Naseem