- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set('Europe/Moscow');
$datetime1 = new DateTime('13-03-01');
$datetime2 = new DateTime('13-04-01');
$datetime3 = new DateTime('13-03-02');
$datetime4 = new DateTime('13-04-02');
$interval = $datetime2->diff($datetime1);
echo $interval->format('%a days'), PHP_EOL; //1 month, 3 days
$interval = $datetime4->diff($datetime3);
echo $interval->format('%a days'); //1 month, 0 days