The problem is when you store the Date in the database we need to change the format from UK to US format. To do this we need to have some function that converts the Date.
I have attached a zip which contain a script that does just that.
And then include the script on the top of your page, and in your insert server behavior, you need to change it to:
date('Y-m-d', strtotime(date_uk_to_american($_POST['fieldset_group_Date'])))