<?php

$created = 1323504315;
$timeleft = 144;
$expires = strtotime("+$timeleft hours", $created);

$compare_text = "created: " . date('m/d/Y H:i A',$created) . ", now: " . date('m/d/Y H:i A') . " > expires:" . date('m/d/Y H:i A',$expires);

if (mktime() > $expires) {
    echo "The time has expired. ($timeleft, $compare_text)";
} else {
    echo "The time has not expired. ($timeleft, $compare_text)";
}

?>

The time has expired. (144, created: 12/10/2011 08:05 AM, now: 12/18/2011 07:17 AM > expires:12/16/2011 08:05 AM)