I found the solution with wp_update_post().
add_action('save_post', 'change_content');
global $post;
global $wpdb;
function change_content($post_id) {
$datefield = get_post_meta($post_id,'acf-event-date',true);
$post_date = date("Y-m-d H:i:s", strtotime($datefield));
$my_post = array();
$my_post['ID'] = $post_id;
$my_post['post_date'] = $post_date;
remove_action('save_post', 'change_content');
wp_update_post( $my_post );
add_action('save_post', 'change_content');
}
Davelee