Making assertions on an instance of DatePeriod with PHPSpec

gistfile1.php

<?php

...

class DeliveryDateService extends ObjectBehavior
{
    ...
    function it_should_return_all_sundays_up_to_the_last_available_delivery_date_when_today_is_a_sunday() 
    {
        //Set the first date in the range to be Sunday, 23rd of June 2013 (16:00)
        $firstDate = new \DateTime();
        $firstDate->setDate(2013, 6, 23);
        $firstDate->setTime(16, 0);
        $this->setFirstDate($firstDate);

        //Set the last date in the range to be 3 months after the first date
        $lastDate = clone $firstDate;
        $lastDate->add(new \DateInterval('P3M'));
        $this->setLastDate($lastDate);

        $dateRange = $this->getSundaysWithinDateRange();
        $dateRange->shouldBeAnInstanceOf('DatePeriod');

        //Unwrap the DatePeriod object so that we can traverse and count
        $unwrappedDateRange = $dateRange->getWrappedSubject();

        foreach ($unwrappedDateRange as $date) {
            if ($date->format('l') != 'Sunday') {
                throw new \Exception(sprintf('Expected "Sunday" but actually got "%s"', $date->format('l')));
            }
            if ($date->getTimestamp() > $lastDate->getTimestamp()) {
                throw new \Exception(sprintf('A timestamp of a date in the DatePeriod is larger than the last available date: %s(%d) > %s(%d)', $date->format('d/m/Y'), $date->getTimestamp(), $lastDate->format('d/m/Y'), $lastDate->getTimestamp()));
            }
            if ($date->getTimestamp() < $firstDate->getTimestamp()) {
                throw new \Exception(sprintf('A timestamp of a date in the DatePeriod is smaller than the first available date: %s(%d) < %s(%d)', $date->format('d/m/Y'), $date->getTimestamp(), $lastDate->format('d/m/Y'), $lastDate->getTimestamp()));
            }
        }
        if (count(iterator_to_array($unwrappedDateRange)) !== 14) {
            throw new \Exception(sprintf('Expected 14 Sundays, but got %d', count(iterator_to_array($unwrappedDateRange))));
        }
    }
}