I think if your attribute is of type DateTime, you should have your users fill-out a dateTime value. This is recommended, because Mendix works with Time zones and Localization. In addition, the DateTime value of a user will be converted by Mendix before it is stored in the database. So in order not to mess up dateTime values, you better have your users fillout a DateTime value in a DateTime attribute, so Mendix can assure you the right TimeZone and localizations configurations are meet.
Good luck with it,