Use the Dunning Terms  Setup window to determine how interest is calculated. The following examples demonstrate each method.
There are 30 days in each month.
There are 360 days in a year.
Due Date 
Document Type 
Total 

01.01.03 
Invoice 
10000 
01.02.03 
Payment 
3000 
Remaining Sum is 100003000 = 
7000 

Interest rate is 7% and the dunning wizard is processed on 01.03.03 7000X7%X60*/360 = 81.6667 * number of days counted since the due date of the invoice and the date in which the dunning wizard is processed. 
Due Date 
Document Type 
Total 

01.01.03 
Invoice 
10000 
01.02.03 
Payment 
3000 
Interest rate is 7 % and the dunning wizard is processed on the 1.3.03 (10000X7%X30/360) + (7000X7%X30/360) = 99.16666 The value of the first parenthesis is the interest that was accumulated until the first payment was created. The value in the second parenthesis is the interest accumulated until the wizard was processed. 