Here's some example:
<?php
$str = '20050616110000';
$now = time();
$year = substr($str,0,4);
$month = substr($str,4,2);
$day = substr($str,6,2);
$hour = substr($str,8,2);
$min = substr($str,10,2);
$sec = substr($str,12,2);
$when = mktime($hour,$min,$sec,$month,$day,$year);
$diff = $now - $when;
$d = floor($diff/(24*60*60));
$remain = $diff%(24*60*60);
$h = floor($remain/(60*60));
$remain = $remain%(60*60);
$m = floor($remain/60);
$s= $remain%60;
echo date("Y/m/d H:i:s",$now)." - $d days, $h hours, $m minutes, $s seconds have passed
since ".date("Y/m/d H:i:s",$when);
?>
The problem would begin when you want to add months and years (as they don't have fixed length.