it does not work! why?
clr-namespace:
is used to create a namespace mapping to custom classes and assemblies and can't be used directly like that in markup extension, it is not a namespace prefix.
Edit: You can't use local types in your assembly directly in your markup extension like that.
First you will have to create a namespace mapping like you have done in your question like this: xmlns:local="clr-namepsace:YOUR_ASSEMBLY_NAME.NAMESPACE_CONTAINING_TYPE_YOU_WANT"
.
Then you can use them in markup extensions like this:
<UserControl DataContext="{Binding Source=local:MyClock.DataBindingDemo}">
// ^
// |
// Notice namespace using here