How to format TimeSpan in XAML

The format string is intended to work on a DateTime, not a TimeSpan.

You could change your code to work with DateTime.Now instead. Your xaml is fine:

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

Update

And from .Net 4 format a TimeSpan as follows:

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