"Old Date" not working in ConfigDate

In ConfigDate the "Old Date" value is not shown (in the "Pick a Date" window).
This value just shows the "actual used" (and not "actual selected") date value and it has been working before 0.35.4.
Strange enough: when putting a string (e.g. replacing self.__value with "Test") into
  self.old_date = gtk.Label(self.__value)
it is working. And the same line for the "New Date" value (self.new_date... 3 lines above) is working with self.__value, too!
No clue what is going wrong here.