- date_default_timezone_set: documentation ( source)
<?php
$date_format = 'Y-m-d H:i:s';
$day = "Sunday 4 week ago";
date_default_timezone_set("Europe/London");
$date = new DateTime($day);
$date->modify('+1 day');
echo $date->format($date_format) . "\n";
date_default_timezone_set("Pacific/Auckland");
$date = new DateTime($day);
$date->modify('+1 day');
echo $date->format($date_format) . "\n";