Calculation of Over Due date for Credit Card Transactions in Expense Module.

Question:
How does the system determine that an Expense Credit Card Transaction is Overdue? 

I know you Define the Overdue Period for Credit Card Transactions on the Business Unit Related, Expenses, Expenses Definition, Business Unit 2 tab .

But how does the system determine when the Overdue Period has been reached.  Ie.. If that value is set at 30 days for example.