PHP time offset

Hi all,
I have inherited a php script that I am hosting on a US server but am trying to get it to work using Australian timezones (its a booking script).
BEfore you ask, changing the timezone of the server is unfortunately not an option.
I have narrowed the problem code down to the below, adding

date_default_timezone_set('Australia/ACT');

PHP:

gets the display part working but when they submit times for booking based on the

$dtDate 

PHP:

variable, it is submitted based on server time.

Any ideas?

<? session_start(); 
date_default_timezone_set('Australia/ACT');
require 'common.php';

$dtDate = date("Y-m-d", mktime(0, 0, 0, date("m")  , date("d")+$_SESSION[OFFSET], date("Y")));

?>

PHP:

 

setTimeZone("2");

function setTimeZone($GMT) {
$timezones = array(
'-12'=>'Pacific/Kwajalein',
'-11'=>'Pacific/Samoa',
'-10'=>'Pacific/Honolulu',
'-9'=>'America/Juneau',
'-8'=>'America/Los_Angeles',
'-7'=>'America/Denver',
'-6'=>'America/Mexico_City',
'-5'=>'America/New_York',
'-4'=>'America/Caracas',
'-3.5'=>'America/St_Johns',
'-3'=>'America/Argentina/Buenos_Aires',
'-2'=>'Atlantic/Azores',// no cities here so just picking an hour ahead
'-1'=>'Atlantic/Azores',
'0'=>'Europe/London',
'1'=>'Europe/Paris',
'2'=>'Europe/Helsinki',
'3'=>'Europe/Moscow',
'3.5'=>'Asia/Tehran',
'4'=>'Asia/Baku',
'4.5'=>'Asia/Kabul',
'5'=>'Asia/Karachi',
'5.5'=>'Asia/Calcutta',
'6'=>'Asia/Colombo',
'7'=>'Asia/Bangkok',
'8'=>'Asia/Singapore',
'9'=>'Asia/Tokyo',
'9.5'=>'Australia/Darwin',
'10'=>'Pacific/Guam',
'11'=>'Asia/Magadan',
'12'=>'Asia/Kamchatka'
);
//date_default_timezone_set($timezones[$GMT]);
custom_timezone_set($timezones[$GMT]);
return true;
}

function custom_timezone_set( $tz )
{
if (function_exists('date_default_timezone_set')) {
date_default_timezone_set( $tz );
}
else {
putenv("TZ=$tz");
}
}

function custom_timezone_get()
{
if (function_exists('date_default_timezone_get')) {
date_default_timezone_get();
}
else {
getenv("TZ");
}
}