Issue with using a factory to retrieve a service in PHP-DI

config.php

<?php

use DiExample\DummyDateCalculatorFactory;

return [
    'DateTime' => new \DateTime(),
    'calculator' => [DI\get('DiExample\DummyDateCalculatorFactory'), 'create'],
];

DummyDateCalculator.php

<?php

namespace DiExample;

class DummyDateCalculator
{
    protected $date;
    protected $secondDate;

    public function __construct(\DateTime $date)
    {
        $this->date = $date;
    }

    public function setSecondDate(\DateTime $secondDate)
    {
        $this->secondDate = $secondDate;
    }

    public function __toString()
    {
        return "Time was initialised at: " . $this->date->format(\DateTime::ATOM);
    }
}

DummyDateCalculatorFactory.php

<?php

namespace DiExample;

use Interop\Container\ContainerInterface;

class DummyDateCalculatorFactory
{
    /**
     * @param $container
     * @return DummyDateCalculator
     */
    public function create(ContainerInterface $container)
    {
        return new DummyDateCalculator($container->get('DateTime'));
    }
}

index.php

<?php

date_default_timezone_set('UTC');

require_once('vendor/autoload.php');

$builder = new DI\ContainerBuilder();
$builder->addDefinitions('config/config.php');
$container = $builder->build();

var_dump($container->get('calculator'));