you need to set Return Format to Ymd because only then you can order dates correct.
and use something like this:
$get_startdate = get_field('my_startdate');
$start_date = (strtotime($get_startdate));
//use $get_startdate to order, and $start_date_pretty to output your date
$start_date_pretty = date_i18n( 'd/m/Y', $start_date );
hope that help you to get what you wish to try
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