To reproduce:
-add RadTimePicker and set its value to null.
-click the arrow to open the pop up and notice that clock header used the default general DateTime pattern to display the text (e.g. 9/18/2013 12:00:00 AM), instead of using the short time pattern displayed (e.g. 12:00 AM)

Workaround:
this.radTimePicker1.Value = null;
this.radTimePicker1.TimePickerElement.PopupOpened+=TimePickerElement_PopupOpened;

private void TimePickerElement_PopupOpened(object sender, EventArgs e)
{
   this.radTimePicker1.TimePickerElement.PopupContentElement.ClockHeaderElement.Text =
        DateTime.Now.ToString(this.radTimePicker1.TimePickerElement.Format, this.radTimePicker1.TimePickerElement.Culture);
}