It includes timestamp by default, but if you set it manually without 'timestamp'...
https://en.wikipedia.org/w/api.php?action=query&list=blocks&bkprop=user
Feb 18 21:27:29 mw1208: #012Notice: Undefined property: stdClass::$ipb_timestamp in /srv/mediawiki/php-1.25wmf16/includes/api/ApiQueryBlocks.php on line 188