AvailabilityResolver - Can you find a bug w/o tests?

AvailabilityResolver.php

<?php

class AvailabilityResolver
{

    /** @var \DateTimeInterface */
    private $dateTime;

    public function __construct(\DateTimeInterface $dateTime = NULL)
    {
        $this->dateTime = $dateTime ?: new \DateTimeImmutable();
    }

    /**
     * Returns a date increased by a given amount of working days.
     * @param int $workingDays
     * @return \DateTimeInterface
     */
    public function date($workingDays)
    {
        $dateTime = new \DateTime();
        $dateTime->setTimestamp($this->dateTime->getTimestamp());
        $dateTime->setTimezone($this->dateTime->getTimezone());

        while ($workingDays-- > 0) {
            $dateTime->modify('+1 day');
            $this->fixWorkingDays($dateTime);
        };

        $this->fixWorkingDays($dateTime);

        return $dateTime;
    }

    private function fixWorkingDays(\DateTime $dateTime)
    {
        do {
            $beforeEncore = clone $dateTime;

            // Check for weekends.
            $weekDay = (int)$dateTime->format('w');
            if ($weekDay === 6) { // Saturday
                $dateTime->modify('+2 days');
            } elseif ($weekDay === 0) { // Sunday
                $dateTime->modify('+1 day');
            }

            // Check for holidays.
            $holidays = $this->holidays();
            $monthDay = $dateTime->format('m-d');
            if (array_search($monthDay, $holidays) !== FALSE) {
                $dateTime->modify('+1 day');
            }
        } while ($beforeEncore != $dateTime); // Intentional weak type comparison.
    }

    private function holidays()
    {
        return [
            '01-01',
            '05-01',
            '05-08',
            '07-05',
            '07-06',
            '09-28',
            '10-28',
            '11-17',
            '12-24',
            '12-25',
            '12-26',
        ];
    }

}

I'm convinced it's impossible find the bug (it really exists there) without tests. It's a scenario where you should always write them!

If you wanna see tests, here they're:

<?php

namespace Tests;

use AvailabilityResolver;
use PHPUnit\Framework\TestCase;

class AvailabilityResolverTest extends TestCase
{

    public function testDateBasic1()
    {
        $date = new \DateTimeImmutable('2017-01-03 0:00:00'); // Tuesday
        $resolver = new AvailabilityResolver($date);

        $this->assertEquals($date->modify('+0 days'),  $resolver->date(0)); // Tuesday + 0 working days = today = Tuesday (+0).
        $this->assertEquals($date->modify('+1 day'),  $resolver->date(1)); // Tuesday + 1 working day = Wednesday (+1).
        $this->assertEquals($date->modify('+2 days'),  $resolver->date(2)); // Tuesday + 2 working days = Thursday (+3).
        $this->assertEquals($date->modify('+3 days'),  $resolver->date(3)); // Tuesday + 3 working days = Friday (+4).
    }

    public function testDateBasic2()
    {
        $date = new \DateTimeImmutable('2017-01-01 0:00:00'); // Sunday
        $resolver = new AvailabilityResolver($date);

        $this->assertEquals($date->modify('+1 day'),  $resolver->date(0)); // Sunday + 0 working days = we need Monday (+1).
        $this->assertEquals($date->modify('+2 days'),  $resolver->date(1)); // Sunday + 1 working day = Tuesday (+2).
        $this->assertEquals($date->modify('+8 days'),  $resolver->date(5)); // Sunday + 5 working days = next Monday (+8).
        $this->assertEquals($date->modify('+9 days'),  $resolver->date(6)); // Sunday + 6 working days => next Tuesday (+9).
        $this->assertEquals($date->modify('+10 days'), $resolver->date(7)); // Sunday + 7 working days = next Wednesday (+10)
    }

    public function testDateWeekend()
    {
        $date = new \DateTimeImmutable('2016-12-31 0:00:00'); // Saturday
        $resolver = new AvailabilityResolver($date);

        $this->assertEquals($date->modify('+2 days'),  $resolver->date(0)); // Saturday + 0 working days = we need Monday (+2).
        $this->assertEquals($date->modify('+3 days'),  $resolver->date(1)); // Saturday + 1 working day = Tuesday (+3).
        $this->assertEquals($date->modify('+4 days'),  $resolver->date(2)); // Saturday + 2 working days = Wednesday (+4).
    }

    public function testDateHoliday()
    {
        $date = new \DateTimeImmutable('2017-04-30 0:00:00'); // Sunday
        $resolver = new AvailabilityResolver($date);

        $this->assertEquals($date->modify('+2 days'),  $resolver->date(0)); // Sunday + 0 working days = wee need Tuesday (Monday is holiday) (+2).
        $this->assertEquals($date->modify('+3 days'),  $resolver->date(1)); // Sunday + 1 working day = Wednesday (+3).
        $this->assertEquals($date->modify('+4 days'),  $resolver->date(2)); // Sunday + 2 working days = Tuesday (+4).
    }

    public function testDateConsecutiveHolidays()
    {
        $date = new \DateTimeImmutable('2017-12-23 0:00:00'); // Saturday
        $resolver = new AvailabilityResolver($date);

        $this->assertEquals($date->modify('+4 days'),  $resolver->date(0)); // Saturday + 0 working days = wee need Wednesday (Monday and Tuesday are holidays) (+4).
        $this->assertEquals($date->modify('+5 days'),  $resolver->date(1)); // Saturday + 1 working day = Thursday (+5).
        $this->assertEquals($date->modify('+6 days'),  $resolver->date(2)); // Saturday + 2 working days = Friday (+6).
        $this->assertEquals($date->modify('+10 days'),  $resolver->date(3)); // Saturday + 3 working days = next Tuesday (Monday is holiday) (+10).
        $this->assertEquals($date->modify('+11 days'),  $resolver->date(4)); // Saturday + 4 working days = next Wednesday (+11).
        $this->assertEquals($date->modify('+12 days'),  $resolver->date(5)); // Saturday + 5 working days = next Thursday (+12).
    }

}