Each survey don't have unique date format, you have to choose date format according to the langage.

Like you say, date is stored in date SQL format, it's only for easy fill date . I think it's best the user can choose is date format.

One thing to do maybe , is to specified default date format for each langage. For example, in french, it's dd/mm/yy , then if i select french langage , date format put by default to dd/mm/yy etc ....