add_filter("gform_post_data", "change_post_date", 10, 3);
     function change_post_date($post_data, $form, $entry){
            if($form["id"] != 1) //FORM ID
            return $post_data;
            $date = $entry[13]; //INPUT ID
            $post_data["post_date"] = date("Y-m-d", strtotime($date));
            return $post_data;
     }