When setting a value in xaml, the TimePicker does not show it in the TextBox, as reported here: http://silverlight.net/forums/t/83044.aspx

Suggested workaround:
Subscribe to the loaded event of your TimePicker and use this code in the eventhandler:

1 private void TimePicker_Loaded(object sender, RoutedEventArgs e)
2 {
3 Dispatcher.BeginInvoke(() =>
4 {
5 DateTime? value = tp.Value;
6
7 tp.Value = null;
8 tp.Value = value;
9 });
10 }