"due date" is not a question. I have edited your post to make the title a question.
In the future, please consider using a Question that can make it clear what you ask without requiring people to open your post, as this will often increase the chances people will respond and make everything clearer for others looking for the same answer.
With many subjects posted, not everyone wants to open a post to find out what the question is.
See also https://www.odoo.com/forum/help-1/meta-why-do-some-questions-get-answers-and-others-do-not-25620
In Odoo for the due date field in invoice tree view currently widget 'remaining_days' is used. That's why it is showing with number of days.
So if you don't want to show like this, you can remove the widget by inheriting the view as below:
1. Inherit the account.move tree view and replace the widget assigned for the 'invoice_date_due' field
Hope this will help you