When invoices are created and payment is not made, Invoices are shown in the Aging report section. This can be accessed by navigating "Accounting section" and "Invoice tab"

This shows each invoice and aging days [that is how many days have passed since invoice is issued and client has not made a payment], Invoice amount, and Ending Balance.

If the number of aging days exceed 30 days, it will be shown as overdue status for the invoice: