пезда
$stack = '2021-08-19 (04:16:12)
2021-08-18 (02:57:07)';
$seconds = 0;
preg_replace_callback('#\((\d{2}):(\d{2}):(\d{2})\)#', function($matches) {
$GLOBALS['seconds'] += mktime($matches[1], $matches[2], $matches[3]);
}, $stack);
echo date('g', $seconds);