<style targettype="{x:type datepickertextbox}">
            <setter property="control.template">
                <setter.value>
                    <controltemplate>
                        <textbox x:name="part_textbox" text="{binding path=selecteddate, stringformat='yyyy-mm-dd', 
     relativesource={relativesource ancestortype={x:type datepicker}}}" />
                    </controltemplate>
                </setter.value>
            </setter>
        </style>