DateTime won't change temporary - php

Question

My whole site is:
date_default_timezone_set('Europe/London');
A user can select their timezone, and in the database it saves as 4.0 (for Dubai)
Now I wish to have so the timezone the user has chosen has an impact on the countdown, that exists on the site. The countdown looks like this:
$today_date = new DateTime('now'); // now
$final_date = new DateTime($date); // a date in the future
$interval = $today_date->diff($final_date); // find the difference
$time_left = $interval->format('starting in %d days, %h hours and %i minutes'); // display it
Above this, i did following:
$userGMT = $user->get_gmt();
date_default_timezone_set($userGMT);
Which does not work correct. It still counts down for Europe/London timezone, and not the one I have chosen.
I tried do echo date('H:i:s'); and can see by the time that the above has affected and it is showing the time for the timezone I have picked.
So that works, but dateTime('now'); doesnt?

Answer 1

What you want to do is add the $userGMT to your currentTime and then subtract your server time. So get the time in seconds then add $userGMT*60*60 - $yourTimeZone*60*60

Answer 2

When the date in the future is generated, it needs to be explicit about the applicable timezone. This demonstrates that the interval is independent of the host's timezone:
<?php
$date = '2012-04-19'; // date in the future
$user_tz = new DateTimeZone('Asia/Dubai');
foreach (array('Europe/London', 'America/Los_Angeles') as $tzname)
{
date_default_timezone_set($tzname);
$today_date = new DateTime('now'); // now
$final_date = new DateTime($date, $user_tz); // future date with explicit tz
$interval = $today_date->diff($final_date); // find the difference
$time_left = $interval->format('start in %d days, %h hours and %i minutes');
echo "$tzname: $time_left\n";
}
Output:
Europe/London: start in 0 days, 11 hours and 53 minutes
America/Los_Angeles: start in 0 days, 11 hours and 53 minutes