This should make the edit button disappear after the time limit has run out (that is, if you reload the page). It replaces the similar code at the bottom of drawmsg.inc. It't not elegant, but it works.
if ( isset($GLOBALS["usr"]) && ($GLOBALS["usr"]->id == $obj->poster_id || $GLOBALS["MOD"]) ) {
if ( $GLOBALS["MOD"] || ( $GLOBALS["EDIT_TIME_LIMIT"] && ( time() < ($obj->post_stamp+($GLOBALS["EDIT_TIME_LIMIT"]*60)) ) ) )
echo '<a class="GenLink" href="post.php?msg_id='.$obj->id.'&'.$ret_n_sid.'">< img src="'.$GLOBALS["fud_img_suffix"].'msg_ed
}
}