function receiveClient(req, success) {
  if( success ) {
    var txt = req.responseText;
    document.getElementById('clientPromos').innerHTML=txt;
  }
}

function changeClient() {
  
  
  var f = new ajax('/page/1/ajax_client_promo.jsp',
                function(req) { receiveClient(req, true); },
                function(req) { receiveClient(req, false); }
            );
  f.doGet();
  window.setTimeout(changeClient, 15000);
}

window.setTimeout(changeClient, 15000);