As of r84905  (going live to Wikimedia within the hour), the cltype
parameter to list=categorymembers is ignored when clsort=timestamp is
set. This change is needed for performance reasons: long-running
queries were causing bug 28291  on Wikimedia wikis.
This does not break backwards compatibility with any released version
of MediaWiki, because cltype is new in 1.17. Functional
cltype=foo&clsort=timestamp behavior was only live on Wikimedia wikis
for about 2-3 weeks.
Roan Kattouw (Catrope)