Hi,
when you’re editing your custom date field, you can see “Display format” on the bottom. This will format the display of the date.
http://www.advancedcustomfields.com/resources/field-types/date-picker/
If you’re setting the format in wp backend, use JS date format, but you can also format the date later in PHP – use the code from example
$date = DateTime::createFromFormat('Ymd', get_field('date_picker'));
echo $date->format('d-m-Y');