Testing methods where DateTime object is used with PhpUnit

declare(strict_types=1);

namespace Api\ExamBundle\Helper;

class ExamDate
{
    public function getLastExamDay()
    {
        $date = $this->getDate('last day of this month');

        if ($date == $this->getDate()) {
            $date = $this->getDate('last day of next month');
        }

        if ($date->format('l') === 'Saturday') {
            $date->modify('-1 day');
        } elseif ($date->format('l') === 'Sunday') {
            $date->modify('-2 days');
        }

        return $date->format('Y-m-d');
    }

    protected function getDate(?string $time = null): DateTime
    {
        return new DateTime($time);
    }
}

declare(strict_types=1);

namespace tests\Api\ExamBundle\Helper;

use Api\ExamBundle\Helper;
use DateTime;
use PHPUnit_Framework_MockObject_MockObject;
use PHPUnit_Framework_TestCase;

class ExamDateTest extends PHPUnit_Framework_TestCase
{
    /**
     * @test
     *
     * @dataProvider getLastExamDayProvider
     */
    public function it_returns_last_exam_day($lastDayOfThisMonth, $today, $lastDayOfNextMonth, $expectedDay)
    {
        /** @var Adaptor|PHPUnit_Framework_MockObject_MockObject $mock */
        $mock = $this->getMockBuilder(ExamDate::class)
            ->disableOriginalConstructor()
            ->setMethods(['getDate'])
            ->getMock();

        $mock
            ->expects($this->at(0))
            ->method('getDate')
            ->willReturn(new DateTime($lastDayOfThisMonth));

        $mock
            ->expects($this->at(1))
            ->method('getDate')
            ->willReturn(new DateTime($today));

        if ($lastDayOfNextMonth) {
            $mock
                ->expects($this->at(2))
                ->method('getDate')
                ->willReturn(new DateTime($lastDayOfNextMonth));
        }

        $this->assertEquals($expectedDay, $mock->getLastExamDay());
    }

    /**
     * 1)
     * Last day of this month : 2015-07-31 (Friday)
     * Today                  : 2015-07-31 (Friday)
     * Last day of this month : 2015-08-31 (Monday) *
     * Result                 : 2015-08-31 (Monday) same day of *
     *
     * 2)
     * Last day of this month : 2015-09-30 (Wednesday)
     * Today                  : 2015-09-30 (Wednesday)
     * Last day of this month : 2015-10-31 (Saturday) *
     * Result                 : 2015-10-30 (Friday) -1 day of *
     *
     * 3)
     * Last day of this month : 2015-04-30 (Thursday)
     * Today                  : 2015-04-30 (Thursday)
     * Last day of this month : 2015-05-31 (Sunday) *
     * Result                 : 2015-15-29 (Friday) -2 days of *
     *
     * 4)
     * Last day of this month : 2015-07-31 (Friday) *
     * Today                  : 2015-07-30 (Thursday)
     * Last day of this month :
     * Result                 : 2015-07-31 (Friday) same day of *
     *
     * 5)
     * Last day of this month : 2015-10-31 (Saturday) *
     * Today                  : 2015-10-16 (Friday)
     * Last day of this month :
     * Result                 : 2015-10-30 (Friday) -1 day of *
     *
     * 6)
     * Last day of this month : 2015-05-31 (Sunday) *
     * Today                  : 2015-05-15 (Friday)
     * Last day of this month :
     * Result                 : 2015-05-29 (Friday) -2 days of *
     */
    public function getLastExamDayProvider(): array
    {
        return [
            ['2015-07-31', '2015-07-31', '2015-08-31', '2015-08-31'],
            ['2015-09-30', '2015-09-30', '2015-10-31', '2015-10-30'],
            ['2015-04-30', '2015-04-30', '2015-05-31', '2015-05-29'],
            ['2015-07-31', '2015-07-30', null, '2015-07-31'],
            ['2015-10-31', '2015-10-16', null, '2015-10-30'],
            ['2015-05-31', '2015-05-15', null, '2015-05-29'],
        ];
    }
}