pw_getLastModified.php

/**
 * Get last modified page modified date from given $id, $parent_id, $templates_id or from all
 *
 * @param bool $id
 * @param bool $parent_id
 * @param bool $templates_id
 * @return mixed|string
 */
function getLastModified($id=false, $parent_id=false, $templates_id=false) {
    if(!is_null($id)) {
        $where = "";
        if(is_bool($id) != true) {
            $where = " WHERE";
            $where .= ($parent_id) ? " parent_id = {$id}" : " id={$id}";
            $where .= ($templates_id) ? " AND templates_id = {$templates_id}" : "";
        }
        $results = wire('db')->query("SELECT MAX(modified) as modified FROM pages{$where}");
        if($results->num_rows > 0) {
            $result = $results->fetch_assoc();
            $search = array(' ', '-', ':');
            $replace = array('', '', '');
            return str_replace($search, $replace, $result['modified']);
        }
    }
    return "";
}

Added $templates_id for template filtering