Format timestamps as UTC in ActivityStreams output. While using local times is legit per spec, it's confusing to have it change around and confuses some clients that don't handle zones right.

lib/activity.php

@@ -704,11 +704,18 @@ class Activity
         return ActivityUtils::child($element, $tag, $namespace);
     }
 
+    /**
+     * For consistency, we'll always output UTC rather than local time.
+     * Note that clients *should* accept any timezone we give them as long
+     * as it's properly formatted.
+     *
+     * @param int $tm Unix timestamp
+     * @return string
+     */
     static function iso8601Date($tm)
     {
         $dateStr = date('d F Y H:i:s', $tm);
         $d = new DateTime($dateStr, new DateTimeZone('UTC'));
-        $d->setTimezone(new DateTimeZone(common_timezone()));
         return $d->format('c');
     }
 }