Working out payments with compund interest and partial advance payments

Hi, I have some data to prepare every month and I want an excel formula which can ease my work.
Example of the data.
There is an Invoice of 980.00 which should due on or before 15th of every month if not paid then 1.5% compound interest will be added until paid. And if amount paid partially before due date then interest calculated after deducting the payment and if paid after the due date then interest to be added on the amount then deducting the paid amount.
it is simple when due is for a month or 2 but it becomes little difficult to calculate if more months.

pls help...

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