code: def _tmdd_datetime_to_iso(dt, include_offset=True, include_seconds=True):
      
      datestring = dt.findtext('date')
      timestring = dt.findtext('time')
      assert len(datestring) == 8
      assert len(timestring) >= 6
      iso = datestring[0:4] + '-' + datestring[4:6] + '-' + datestring[6:8] + 'T' \
          + timestring[0:2] + ':' + timestring[2:4]
      if include_seconds:
          iso += ':' + timestring[4:6]
      if include_offset:
          offset = dt.findtext('offset')
          if offset:
              assert len(offset) == 5
              iso += offset[0:3] + ':' + offset[3:5]
          else:
              raise Exception("TMDD date is not timezone-aware: %s" % etree.tostring(dt))
      return iso
docs: dt is an xml Element with <date>, <time>, and optionally <offset> children.
      returns an ISO8601 string