Windows Phone 8.1 TextBlock StringFormat support

"Unsupported".xaml

<Style TargetType="controls:Clock">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="controls:Clock">
                <TextBlock Text="{Binding DateTime, 
                    StringFormat=T,
                    RelativeSource={RelativeSource TemplatedParent}}" />
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Supported.xaml

<TextBlock Text="{Binding Minutes, StringFormat='00'}" />

<TextBlock Text="{Binding Seconds, StringFormat='00'}" />