DateFormatter.php

<?php

namespace WP;

use DateTime;

interface DateFormatterInterface
{
    public function set(DateTime $newDate) : DateTime;
    public function transformTo(FormatInterface $newFormat) : string;
}

interface FormatInterface
{
    public function convert(DateTime $dateToConvert) : string;
}

class PolishDateFormat extends \IntlDateFormatter implements FormatInterface
{
    public function __construct(
        $locale   = 'pl_PL',
        $datetype = \IntlDateFormatter::LONG,
        $timetype = \IntlDateFormatter::NONE,
        $timezone = 'Europe/Warsaw',
        $calendar = \IntlDateFormatter::GREGORIAN,
        $pattern  = null
    ) {
        parent::__construct($locale, $datetype, $timetype, $timezone, $calendar, $pattern);
    }

    public function convert(DateTime $dateToConvert) : string
    {
        return $this->format($dateToConvert);
    }
}

class DateFormatter implements DateFormatterInterface
{
    private $date;

    public function set(DateTime $newDate) : DateTime
    {
        return $this->date = $newDate;
    }

    public function transformTo(FormatInterface $newFormat) : string
    {
        return $newFormat->convert($this->date);
    }
}

DateFormatterSpec.php

<?php

namespace spec\WP;

use WP\{
    DateFormatter,
    PolishDateFormat
};
use PhpSpec\ObjectBehavior;
use Prophecy\Argument;

use DateTime;

class DateFormatterSpec extends ObjectBehavior
{
    private $dateFormatter;

    public function __construct()
    {
        $this->dateFormatter = new DateFormatter();
    }

    public function it_DateFormatter_should_return_valid_DateTime_type()
    {
        $this->set(new DateTime('2017-03-14'))->shouldHaveType(DateTime::class);
    }

    public function it_should_translate_given_datetime_to_formatted_string()
    {
        $this->set(new DateTime('2017-03-14'));
        $this->transformTo(new PolishDateFormat())->shouldBeString('14 marca 2017');
    }
}