I've created an approval workflow but I want to be able to restrict items from being submitted if it misses a deadline date?
Sorry if that doesn't make sense!
You can create a
Column Validation to submit your form in case the formula is valid
Column Validation> add your formula as you prefer.
You want to submit the form, in case the due date is greater than today! , so the formula would be
=[Due Date] > today()
When you try to set [Due Date] less than Today, The form will not be submitted and you will get the
User Message that you have set in the Column validation