hi, try if this works: (sort by date-field instead of header)
foreach( $repeater as $key => $row ) {
$thedate = $row['your_date_field'];
$column_id[ $key ] = strtotime($thedate);
}
array_multisort( $column_id, SORT_DESC, $repeater );
foreach( $repeater as $row ){
i assume date field is part of the repeater.
replace your_date_field with name of the date field. (part of my code)