In the API JSON output for action=pagetriagestats, pagetriagestats.stats.unreviewedarticle.oldest should really be in ISO 8601 format, i.e. "YYYY-MM-DDTHH:mm:ss.sssZ". MediaWiki-format date strings require special handling and can't be directly parsed by the JavaScript Date class. In general, the core MW API outputs ISO 8601 when it's dealing with dates / timestamps and generating JSON output, the current behaviour of this extension is abnormal relative to that.

http://www.ecma-international.org/ecma-262/6.0/#sec-date-time-string-format