public Date getSince() {
        return since;
    }
Returns the since date to use to filter the messages received during that time. 
In other words, only the messages received since the datetime specified will be 
included in the history.

@return the since date to use to filter the messages received during that time.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/HistoryRequest.java