Edd is correct, there are filters. You could use load value or you can set the default value using load_field. Here is some example code
add_filter('acf/load_field/name=YOUR_FILED_NAME', 'my_acf_default_date');
function my_acf_default_date($field) {
$field['default_value'] = date('Ymd');
return $field;
}