I usually use the display Return Format setting when I create the ACF date field.
If you want to format this in some other way then you still need to set the return format because the functions you use to format a date does not see the default ACF formatting as a valid date.
This is the page that tells you what the valid values look like http://php.net/manual/en/datetime.formats.date.php
This page explains the values that you can enter into ACF to format the date in a different way http://php.net/manual/en/function.date.php
