I remember graylog switching from Joda time to Java time a while ago, so it looks like documetation of pipeline function parse_date is outdated, because it still links to joda time help:
parse_date(value, pattern, [locale], [timezone]) : DateTime` Parses a date string using the given date format
Parameter Type Required Description
--- --- --- ---
value String Date string to parse
pattern String The pattern to parse the date with, see http://www.joda.org/joda-time/apidocs/org/joda/time/format/DateTimeFormat.html
locale String The locale to parse the date with, see https://docs.oracle.com/javase/8/docs/api/java/util/Locale.html
timezone String The timezone to apply to the date, defaults to UTC