$date1 = new \DateTime('2014-09-01 12:35:54');
$date2 = new \DateTime('2014-09-03 06:14:27');
$diff = $date2->diff($date1);
// разница в секундах
$seconds = ($diff->y * 365 * 24 * 60 * 60) +
($diff->m * 30 * 24 * 60 * 60) +
($diff->d * 24 * 60 * 60) +
($diff->h * 60 * 60) +
($diff->i * 60) +
$diff->s;