Код сравнения даты не может найти даты августа и сентября

У меня есть простой фрагмент кода для сравнения дат и подсчета продуктов, он работает для всех моих месяцев, кроме августа и сентября. Я прошел через это дюжину раз, все выглядит одинаково для всех месяцев, но даты августа и сентября не найдены, другие даты проверены.

Извините за хромой код, я еще новичок.

заранее спасибо

<?php

//$ship_date = 20170420;
//$ship_date = 20170520;
//$ship_date = 20170620;
//$ship_date = 20170720;
//$ship_date = 20170820;
//$ship_date = 20170920;
//$ship_date = 20171020;
//$ship_date = 20171120;
$ship_date = 20171220;

$apr = '1';
$may = '1';
$june = '1';
$july = '1';
$august = '1';
$september = '1';
$october = '1';
$nov = '1';
$decem = '1';

$valid = "Y";

$case_qty = 1;

$apr_start = 20170401;
$apr_end = 20170430;
$may_start = 20170501;
$may_end = 20170531;
$june_start = 20170601;
$june_end = 20170630;
$july_start = 20170701;
$july_end = 20170731;
$augmonth_start = 20170801;
$augmonth_stop = 20170831;
$september_start = 20170901;
$september_stop = 20170930;
$october_start = 20171001;
$october_end = 20171031;
$nov_start = 20171101;
$nov_end = 20171131;
$decem_start = 20171201;
$decem_end = 20171231;
$jan_start = 20180101;
$jan_end = 20180131;
$feb_start = 20180201;
$feb_end = 20180228;
$mar_start = 20180301;
$mar_end = 20180331;

if ($valid == "Y")
{

if (($ship_date >= $apr_start) && ($ship_date <= $apr_end))
{
echo "Sold apr<br />";

$apr = $apr + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
//$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $may_start) && ($ship_date <= $may_end))
{
echo "Sold may<br />";

$may = $may + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
//$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $june_start) && ($ship_date <= $june_end))
{
echo "Sold june<br />";

$june = $june + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
//$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $july_start) && ($ship_date <= $july_end))
{
echo "Sold july<br />";

$july = $july + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
//$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $augmonth_start) && ($ship_date <= $augmonth_end))
{
echo "Sold august <br />";

$august = $august + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;

$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $september_start) && ($ship_date <= $september_end))
{
echo "Sold sept <br />";

$september = $september + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;

$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $october_start) && ($ship_date <= $october_end))
{
echo "Sold oct<br />";

$may = $october + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
//$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $nov_start) && ($ship_date <= $nov_end))
{
echo "Sold nov<br />";

$nov = $nov + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
//$nov = 0;
$decem = 0;
} elseif (($ship_date >= $decem_start) && ($ship_date <= $decem_end))
{
echo "Sold decem<br />";

$decem = $decem + $case_qty;

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
//$decem = 0;
} elseif (($ship_date >= $jan_start) && ($ship_date <= $jan_end))
{
echo "Sold jan<br />";

$jan = $jan + $case_qty;

//$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $feb_start) && ($ship_date <= $feb_end))
{
echo "Sold feb<br />";

$feb = $feb + $case_qty;

$jan = 0;
//$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} elseif (($ship_date >= $mar_start) && ($ship_date <= $mar_end))
{
echo "Sold mar<br />";

$mar = $mar + $case_qty;

$jan = 0;
$feb = 0;
//$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;
} else
{

echo "No month selected!!!!!!  " . $ship_date . "<br />";

$jan = 0;
$feb = 0;
$mar = 0;
$apr = 0;
$may = 0;
$june = 0;
$july = 0;
$august = 0;
$september = 0;
$october = 0;
$nov = 0;
$decem = 0;}
}
echo "Month Totals: " . $jan . " " . $feb . " " . $mar . " " . $apr . " " . $may . " " . $june . " " . $july .
" " . $august . " " . $september . " " . $october . " " . $nov . " " . $decem . "<br />";

?>