In the review screen the app has a default due date button that allows the quick entry of a due date. By default the value of that button is "+30" which means the due date will be set to 30 days after the invoice date. The paid date offers a similar functionality with the default value being "+1" as shown in the screenshot below.
The default values for these buttons can be configured in the company preferences under:
Clicking the default due date button will now lead to the due date being set to 20 days after the end of the month.