Related Questions:
Where can I choose my Document Due Date?
Why is the Due Date on my Document blank?
Can I add a Duet Date to a Document I've already created?
Answer:
When Creating a New Document
When creating a Document you are able to choose the Document due date from the creation page.
When Adding to an Existing Document
You can also add in a Due Date once the Document has already been created. This can be done from the Document info tab when the Document is open.