function PickupMonthSelect_onchange() {

    var ddlDrop = document.forms[0].elements['ctl00_ContentPlaceHolder1_tbcMain_tpCarRental_CarRentalSearch1_ddlDropoffMonth'];
    var ddlCollect = document.forms[0].elements['ctl00_ContentPlaceHolder1_tbcMain_tpCarRental_CarRentalSearch1_ddlPickupMonth'];
    if (ddlDrop.selectedIndex < ddlCollect.selectedIndex) {
        ddlDrop.selectedIndex = ddlCollect.selectedIndex;
    }
}