When you use the client side date validation it will allow you to select the appropriate format for the date and also has an option to reformat the date to the type you desire. This reformating is done when the form is submitted and the validations run.
The server side validation for the date is just to ensure the user enters it in a particular fashion, you can use this and a validation show if to display a message to the user to indicate the format the date must be entered.
Like Brad mentioned I think spry also has this type of validation.