<?php
ini_set('date.timezone', 'Europe/Lisbon');
ini_set('intl.default_locale', 'en_US');
$cal = new IntlGregorianCalendar(2013, 4 /* May */, 1, 0, 0, 0);
$time = $cal->getTime();
var_dump($time, $time / 1000 == strtotime('2013-05-01 00:00:00')); //true
The above example will output:
float(1367362800000)
bool(true)