Prefixes the slug with the ``published_on`` date.

def _create_date_slug(self):
        """Prefixes the slug with the ``published_on`` date."""
        if not self.pk:
            # haven't saved this yet, so use today's date
            d = utc_now()
        elif self.published and self.published_on:
            # use the actual published on date
            d = self.published_on
        elif self.updated_on:
            # default to the last-updated date
            d = self.updated_on
        self.date_slug = u"{0}/{1}".format(d.strftime("%Y/%m/%d"), self.slug)