the question you have posted is more suitable to the database forum i think.
do usrcomments and classmembers tables have any fields in common? you might want to request fields specifically by field name otherwise one table might over write the fields in the other if there are any field name collisions.
you might also want to given a different alias to the DATE_FORMAT result...something other than dateadd...maybe formatted_date or something.
other wise:
<?
$q4 = "select * from usrcomments, class_members where usrcomments.commentid = '$_GET[id]' and usrcomments.commentid = class_members.MemberID and sect = '1' ORDER BY id DESC LIMIT 15";
$r4 = mysql_query($q4) or die(mysql_error());
$row = mysql_fetch_assoc($r4);
$parts = explode(' ', $row['dateadd']);
$date_parts = explode('-', $parts[0]);
$time_parts = explode(':', $parts[1]);
$stamp = mktime($time_parts[0],
$time_parts[1],
$time_parts[2],
$date_parts[1],
$date_parts[2],
$date_parts[0],
);
echo date('M-jS-Y h:i:s A');
?>