In the Billy system by default the due date in not set on a calendar. So how do users do that?
The due date is set in the "Due date" field in the table on the top right of the screen.
However this field normally does not show a calendar. It's displays accounting payment terms. So if the desire is to be be paid in 30 days then the user selects "Net 30". Then the system will calculate the calendar date to display on the Invoice and the corresponding email (if sent.)
"Net 30" is the default because this is historically the most common business terms. However there are many other preset terms, as well as the ability to create custom terms.
If you want to set the due date by the calendar select "Custom date..." and a calendar will become available.