Exporting to Excel from DataTables and add messageTop from datepicker

Hi, this works ok but only with the originally loaded date, but when I change the date and when I press the excel button, the messageTop gives the initial date. How dynamically to take a value from txtFromDate and txtToDate and show in messageTop. Ty

function CreateDataTable(data) {
** var fDate = $('#<%= txtFromDate.ClientID %>').val();
var tDate = $('#<%= txtToDate.ClientID %>').val();**

        table = $('#tblUsers').dataTable({
            'lengthChange': true,
            'data': data,
            'autoWidth': true,
            'paging': true,  // Table pagination                
            'ordering': true,  // Column ordering
            'info': false,  // Bottom left status text
            'searching': true,
            'responsive': true, // https://datatables.net/extensions/responsive/examples/

            // Text translation options
            // Note the required keywords between underscores (e.g _MENU_)
            oLanguage: {
                sSearch: 'Пребарај:',
                sLengthMenu: '_MENU_ Записи по страна',
                info: 'Showing page _PAGE_ of _PAGES_',
                zeroRecords: 'Nothing found - sorry',
                infoEmpty: 'No records available',
                infoFiltered: '(filtered from _MAX_ total records)'
            },
            // Datatable Buttons setup
            dom: '<"html5buttons"B>lTfgitp',
            buttons: [
                {
                    extend: 'csv',
                    className: 'btn-sm',
                    title: $('#docTitle').text(),
                    charset: 'UTF-8',
                    bom: true,
                },
                {
                    extend: 'excel',
                    className: 'btn-sm',
                    charset: 'UTF-8',
                    bom: true,
                    title: $('#docTitle').text(),
                  **  messageTop: 'Дата Од: ' + fDate + ' ' + 'Дата До: ' + tDate,**
                    exportOptions: {
                        columns: ':not(:last-child)',
                    }
                },
                { extend: 'pdf', className: 'btn-sm', title:  $('#docTitle').text(), bom: true },
                { extend: 'print', className: 'btn-sm', charset: 'UTF-8', title:  $('#docTitle').text(), bom: true }
            ],