hi, i think something like that should work
add_filter('acf/load_value/name=repeater_termin', 'my_acf_load_value', 10, 3); //repeater_name
function my_acf_load_value( $rows)
{
foreach( $rows as $key => $row ) {
$thedate = $row['termin_date']; //datepicker_fieldname
$column_id[ $key ] = strtotime($thedate);
}
array_multisort( $column_id, SORT_ASC, $rows );
return $rows;
}