As described in Flexibly Parse Date and the documentation at http://docs.graylog.org/en/2.2/pages/extractors.html, Extractors only work on text (string) fields, but “timestamp” is not a string.
As described in Flexibly Parse Date and the documentation at http://docs.graylog.org/en/2.2/pages/extractors.html, Extractors only work on text (string) fields, but “timestamp” is not a string.