For consistency, prop=categories&clprop=timestamp now returns timestamps in ISO 8601 format (e.g. 2008-08-23T18:05:46Z) rather than MediaWiki's internal timestamp format (e.g. 20080823180546) as of r40161 [1]. All timestamps that the API outputs are now in ISO 8601 format.

Roan Kattouw (Catrope)

[1] http://svn.wikimedia.org/viewvc/mediawiki?view=rev&revision=40161