næste måned i kalender

man kan vælge forrige og næste måned vha klik på pilene til venstre og højre.

Men lige så snart den når october, så kan man ikke trykke videre til næste måned. Jeg kan ikke lige se hvorfor?

<?php

function this_timestamp() {
    return mktime(0, 0, 0, $_GET['month'], $_GET['day'], $_GET['year']);
}

function weekday() {
    $day = date("w", this_timestamp());
    return ($day ? $day : 7);
}

if (!isset($_GET['year'])) $_GET['year'] = date("Y");
if (!isset($_GET['month'])) $_GET['month'] = date("n");
$_GET['day'] = 1;


echo "<table  border=1 cellspacing=0 cellpadding=0 align=center>\n";

echo "<tr>\n";

echo "<td style=\"font-size: 10 px\" colspan=8 align=center >"; ?><a href="vis_kalender.php?man_start=<? echo(date("d.m.Y",$start)); ?>&man_slut=<? echo(date("d.m.Y",$slut)); ?>&man_start_tid=<? echo(date("H",$start)); ?>&man_slut_tid=<? echo(date("H",$dag_start_slut)); ?>&day=<? echo(date("j",this_timestamp())); ?>&month=<? echo(date("n",this_timestamp()-(date("t",this_timestamp())*86400))); ?>&year=<? echo(date("Y",this_timestamp()-(date("t",this_timestamp())*86400))); ?>" >&larr; </a>&nbsp;<? echo(date("M",this_timestamp())); ?><a href="vis_kalender.php?man_start=<? echo(date("d.m.Y",$start)); ?>&man_slut=<? echo(date("d.m.Y",$slut)); ?>&man_start_tid=<? echo(date("H",$start)); ?>&man_slut_tid=<? echo(date("H",$dag_start_slut)); ?>&day=<? echo(date("j",this_timestamp())); ?>&month=<? echo(date("n",(this_timestamp()+(date("t",this_timestamp())*86400)))); ?>&year=<? echo(date("Y",this_timestamp()+(date("t",this_timestamp())*86400))); ?>" >&rarr;</a></td>

<?
echo "</tr><tr>\n";

echo "<td style=\"font-size: 10 px\" >U&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\" >Ma&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">Ti&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">On&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">To&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">Fr&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">Lø&nbsp;</td>\n";
echo "<td style=\"font-size: 10 px\">Sø&nbsp;</td>\n";

echo "</tr><tr>\n";

if (weekday() > 1) echo "<td  style=\"font-size: 10 px\" colspan=\"". (weekday()) ."\" >".(date("W",this_timestamp()))."&nbsp;</td>\n";

$days = date("t", this_timestamp());

for ($_GET['day'] = 1; $_GET['day'] <= $days; $_GET['day'] ++) {
  if(weekday() == 1) echo"<td  style=\"font-size: 10 px\" align=\"center\" >".((weekday() == 1) ? (date("W",this_timestamp())) : "&nbsp;")."</td>\n";
    echo "<td  style=\"font-size: 10 px\" align=\"center\" >". $_GET['day'] ."</td>\n";
  if (weekday() == 7) echo "</tr><tr>\n";
}

?>