docs: 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)