The value of If-Modified-Since HTTP header, or None.

        This header is represented as a `datetime` object.

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

        This header is represented as a `datetime` object.
        """
        return self._http_date(self.headers.get(hdrs.IF_MODIFIED_SINCE))