Something like this will get it into a time stamp...and then you can do your calculations with ease, and reformat...
<?php
$str = "20070617T193500";
list($date, $time) = explode("T",$str);
?>
Even easier:
$timestamp = strtotime( $str );
http://us2.php.net/manual/en/function.strtotime.php
$oneMinute = 60; // seconds
$oneHour = $oneMinute * 60;
$oneDay = $oneHour * 24;
echo '5 minutes ago: ' . date( 'm/d/Y h:i:s', $timestamp - ( $oneMinute * 5 ));
echo '5 hours ago: ' . date( 'm/d/Y h:i:s', $timestamp - ( $oneHour * 5 ));
echo '5 days ago: ' . date( 'm/d/Y h:i:s', $timestamp - ( $oneDay * 5 ));
etc., etc., etc.
thnx,
Chris