No need to use date_default_timezone_set
for the whole script, just specify the timezone
you want with a DateTime object:
$now = new DateTime(null, new DateTimeZone('America/New_York'));
$now->setTimezone(new DateTimeZone('Europe/London')); // Another way
echo $now->format("Y-m-d\TH:i:sO"); // something like "2015-02-11T06:16:47+0100" (ISO 8601)