Очередь заданий в виде таблицы SQL с несколькими потребителями (PostgreSQL)

$times = array('17:29:53','16:00:32');

$totaltime = '';
foreach($times as $time){
        $timestamp = strtotime($time);
        $totaltime += $timestamp;
}

$average_time = ($totaltime/count($times));

echo date('H:i:s',$average_time);