Hi Boyko,
The idea is you get the Project from the database in the case of the edit, or not in case it is a new one.
In any case, your Due Date input value is associated to the GetProjectById.List.Current.Project.DueDate date, so when the user selects a due date on the screen s/he will be editing the GetProjectById.List.Current.Project.DueDate variable and therefore it will hold the value that you validate your form against, and the value you are going to insert in the database if forms validations are ok.
Kind Regards,
João Marques