void BillingEngine_EinvNodeAdding(object sender, MercatorUi.Engine.Gescom.BillingEngine.EinvNodeAddedEventArgs e)
{
    MercatorUi.Engine.Gescom.BillingEngine billingEngine = (MercatorUi.Engine.Gescom.BillingEngine)sender;
    if (e.Node.Name == "cac:OrderReference")
    {
        XmlNode n = billingEngine.EinvAddNode(e.eInvoiceEnum, e.ParentNode, "cac:InvoicePeriod");
        billingEngine.EinvAddNode(e.eInvoiceEnum, n, "cbc:StartDate", Convert.ToDateTime(billingEngine.PIEDS["date_1"]));
        billingEngine.EinvAddNode(e.eInvoiceEnum, n, "cbc:EndDate", Convert.ToDateTime(billingEngine.PIEDS["date_2"]));
    }
}