BonusCalculatorTest.php

<?php

class BonusCalculatorTest extends TestCase
{
    public function test_isExpired_should_be_true()
    {
        $target = $this->createPartialMock(BonusCalculator::class, [
            'getDateTime',
        ]);

        // mock DateTime::getToday() return value....
        $datetime = $this->createMock(DateTime::class);
        $datetime->method('getToday')->willReturn('2015-12-26');

        // inject mock object: DateTime
        $target->method('getDateTime')->willReturn($datetime);

        $this->assertTrue($target->isExpired());
    }
}