I lent the amount of 50 000.00 to somebody. We agreed that the
following repayments would be made on the dates below:

Payments and due dates a

2012/05/05 : 10 000.00
2012/06/05 : 10 000.00
2012/07/05 : 10 000.00
2012/08/05 : 10 000.00
2012/09/05 : 10 000.00

Only the following repayments were made:

2012/06/13 : 10 000.00
2012/07/13 : 10 000.00
2012/09/11 : 5 000.00
2012/10/13 : 5 000.00
2012/11/21 : 5 000.00
2013/07/28 : 2 000.00

So, 13 000.00 is stiill outstanding (excl interest).

With regards to non-payments the borrower agreed to the following
conditions:

"In the event that the balance not be paid by the due dates, such
amount shall bear interest at a rate equal to 12% per annum,
compounded monthly in arrears, and reckoned from the effective date
until the date of final payment of the purchase price, both dates
inclusive. The interest due from time to time will be paid monthly on
the first day of each and every month."

How do I setup the above loan in Excel so that I can calculate the
interest that is due on the amount outstanding ?

Thanks.