Your default due date setting is used when you create an invoice, although it may be changed when manually creating an invoice. The default due date is used in automated invoicing to set the due date. 

For instance, if your invoice is created on April 1, 2003, and the default due date setting is "10", then the due date on your invoice will be April 11, 2003.

To setup your default due date:
1. Click on preferences
2. Change the setting in the line for Default Due Date
3. Click Update Preferences button to save your changes.