docs: def duration_to_text(self, duration):
          """
          Return the textual representation of the given `duration`. The duration can either be a tuple of
          :class:`datetime.time` objects, or a simple number. The returned text will be either a hhmm-hhmm string (if the
          given `duration` is a tuple) or a number.
          """
          if isinstance(duration, tuple):
              start = (duration[0].strftime(self.ENTRY_DURATION_FORMAT)
                       if duration[0] is not None
                       else '')
  
              end = (duration[1].strftime(self.ENTRY_DURATION_FORMAT)
                     if duration[1] is not None
                     else '?')
  
              duration = '%s-%s' % (start, end)
          else:
              duration = six.text_type(duration)
  
          return duration