def if_unmodified_since(self) -> Optional[datetime.datetime]:
        """The value of If-Unmodified-Since HTTP header, or None.

        This header is represented as a `datetime` object.
        """
        return self._http_date(self.headers.get(hdrs.IF_UNMODIFIED_SINCE))
The value of If-Unmodified-Since HTTP header, or None.

        This header is represented as a `datetime` object.