public function calculationWorks($method, $dateTime, $interval, $expected)
{
$timestamp = $dateTime->getTimestamp();
$helper = new DateHelper();
$result = $helper->{$method}($dateTime, $interval);
$this->assertEquals($timestamp, $dateTime->getTimeStamp(), 'DateTime should not be modified');
$this->assertEquals($expected, $result->format('Y-m-d H:i:s'));
}