I think it's important to mention with the DateTime class that if you're trying to create a system that should store UNIX timestamps in UTC/GMT, and then convert them to a desired custom time-zone when they need to be displayed, using the following code is a good idea:
<?php
date_default_timezone_set('UTC');
?>
Even if you use something like:
<?php
$date->setTimezone( new DateTimeZone('UTC') );
?>
... before you store the value, it doesn't seem to work because PHP is already trying to convert it to the default timezone.