Default Period for Invoice Due Date
The setting is in: Administration Center > Features > General.
Default Period for Invoice Due Date defines the default time period of the invoice due date.
Click Default Period for Invoice Due Date to set the value.
The number represents days. If the Date Issued field is filled on the invoice, the due date is automatically calculated based on this setting.
When you change the settings, be sure to click Save. For more information, go to Features.