code: def isoformat(self):
          
          # return "%04d-%02d-%02d" % (self._year, self._month, self._day)
          return "%s-%s-%s" % (str(self._year).zfill(4), str(self._month).zfill(2), str(self._day).zfill(2))
docs: Return the date formatted according to ISO.
  
          This is 'YYYY-MM-DD'.
  
          References:
          - http://www.w3.org/TR/NOTE-datetime
          - http://www.cl.cam.ac.uk/~mgk25/iso-time.html