- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set('Europe/London');
$dts = array();
$dts[] = new DateTime('2013-07-09 10:01:02 America/New_York');
$dts[] = new DateTime('10:01:02');
$dts[] = new DateTime('10:01:02 America/New_York');
$dts[] = new DateTime('10:01:02 Australia/Sydney');
$dts[] = new DateTime('10:01:02 UTC');
foreach ( $dts as $dt )
echo $dt->format('c - e').PHP_EOL;