Are you trying to get the number of days between the two dates?
if so:
$date_1 = date_create(get_field('retreat_date_from'));
$date_2 = date_create(get_field('retreat_date_to'));
$diff = date_diff($date_1,$date_2);
This will show you the output:
echo "<pre>";
echo $diff->days;
echo "</pre>";