def isoformat(self):
<EOL>return "<STR_LIT>" % (str(self._year).zfill(<NUM_LIT:4>), str(self._month).zfill(<NUM_LIT:2>), str(self._day).zfill(<NUM_LIT:2>))<EOL>
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