'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'

def isoformat(self):

    return ('%04d-%02d-%02d' % (self._year, self._month, self._day))
