<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>