date('Y-m-d H:i',strtotime('+1 hour +20 minutes',strtotime($start)));
$date = new DateTime('2006-12-12 15:00');
$date->modify('+1 hour');
echo $date->format('Y-m-d'); // 2006-12-12 16:00
$minutes_to_add = 5;
$time = new DateTime('2011-11-17 05:05');
$time->add(new DateInterval('PT' . $minutes_to_add . 'M'));
$stamp = $time->format('Y-m-d H:i');
<?php
$date = new DateTime();
echo $date->getTimestamp(). "<br>";
$date->add(new DateInterval('PT674165S')); // adds 674165 secs
echo $date->getTimestamp();
?>