hi bosoxbill,
I solved this with
<?php
$date_from = get_field( 'retreat_date_from' );
$date_to = get_field( 'retreat_date_to' );
$date_1 = new DateTime(date('Y-m-d', strtotime($date_from)));
$date_2 = new DateTime(date('Y-m-d', strtotime($date_to)));
$days = $date_1->diff($date_2)->days;
?>