Code:
<?
$date = 8-6-2008
$d = explode('-',$date)
$daysAgo = round( ( mktime(0,0,0,$d[0],$d[1],$d[2]) - time() ) / 86400 )
?>
Then you can use a switch case to see how many days ago it was.
HTH. If you have the seconds minutes and hours it can be more accurate