public TimeSpan MyTime { get; set; }

    public Window2()
    {
        InitializeComponent();
        MyTime = DateTime.Now.TimeOfDay;
        DataContext = this;
    }

<TextBlock Text="{Binding MyTime,StringFormat=HH:mm}"/>

<TextBlock>
    <TextBlock.Text>
        <MultiBinding StringFormat="{}{0}:{1}">
            <Binding Path="MyTime.Hours"/>
            <Binding Path="MyTime.Minutes"/>
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

<TextBlock Text="{Binding MyTime,StringFormat=HH:mm}"/>

<TextBlock Text="{Binding MyTime,StringFormat=hh\:mm}"/>

<TextBlock Text="{Binding MyTime,StringFormat=`Time values are {0:hh\:mm}`}"/>

Binding="{Binding Time, Mode=TwoWay, StringFormat={0:h\:mm}}"