If the forms with this validation get opened and saved at a later time, you may want to add another field to prevent the validation from running.
Ex: First time use, the user puts 6 days from now, but someone opens the form 4 days from now, the validation on that field will not allow them to save the form without fixing the date.
If you don't have to worry about that type of event occuring, then the validation works perfectly.
If you do or can have someone open the form at a later date and resave, you may want to add another field, a boolean field (true/false) that is defaulted to false and changes to true when the form gets submitted.
The validation for the due date should check that boolean field to see if it is false. If it is false, the validation would run.
Once the form is submitted, the validation won't run since the boolean field is true.