I have written an extenstion for the namespace above, that lets you pick out the timestamp of messages that are sended in delay. Like history messages that may get transmitted if connecting to a chatroom. We use it in our open source Learning Management System OLAT exactly for this reason. May it gets it into the upcoming release of xiff 2.0.0.
You get the extension here: http://www.olat.org/downloads/tools/extension.zip