Your problem is related to the answer: http://stackoverflow.com/a/3594927/3006165https://stackoverflow.com/a/3594927/3006165
The error is due to date having auto_now_add=True (or auto_now=True). As the value is automatic, it's not editable, so it's not in the form
According to the documentantion:
The fields option ... may contain callables only if they are listed in readonly_fields.