Clock

<?php

class Clock {
    private $minutes;

    /**
     * @param int $hour
     * @param int $minutes
     */
    public function __construct($hour, $minutes = 0)
    {
        $this->minutes = $hour * 60 + $minutes;
    }

    /**
     * Returns a new Clock incremented by $minutes
     *
     * @param int $minutes
     * @return Clock
     */
    public function add($minutes)
    {
        return $this->build($this->minutes + $minutes);
    }

    /**
     * Returns a new Clock deincremented by $minutes
     *
     * @param int $minutes
     * @return Clock
     */
    public function sub($minutes)
    {
        return $this->build($this->minutes - $minutes);
    }

    /**
     * Returns the string representation of the receiver
     *
     * @return string
     */
    public function __toString()
    {
        return sprintf('%02d:%02d', $this->minutes / 60, $this->minutes % 60);
    }

    /**
     * @param $minutes
     * @return Clock
     */
    private function build($minutes)
    {
        if ($minutes < 60) {
            $minutes += 24 * 60;
        }

        $hour = floor($minutes / 60) ?: 0;
        return new Clock($hour < 24 ? $hour : $hour - 24, $minutes % 60);
    }
}

ClockTest

<?php

require_once __DIR__ . '/Clock.php';

class ClockTest extends PHPUnit_Framework_TestCase {
    public function testOnTheHour() {
        $clock = new Clock(8);

        $this->assertEquals('08:00', $clock->__toString());
    }

    public function testPastTheHour() {
        $clock = new Clock(11, 9);

        $this->assertEquals('11:09', $clock->__toString());
    }

    public function testAddingAFewMinutes() {
        $clock = new Clock(10);

        $clock = $clock->add(3);

        $this->assertEquals('10:03', $clock->__toString());
    }

    public function testAddingOverAnHour() {
        $clock = new Clock(10);

        $clock = $clock->add(61);

        $this->assertEquals('11:01', $clock->__toString());
    }

    public function testWrapAroundAtMidnight() {
        $clock = new Clock(23, 30);

        $clock = $clock->add(60);

        $this->assertEquals('00:30', $clock->__toString());
    }

    public function testSubtractMinutes() {
        $clock = new Clock(10);

        $clock = $clock->sub(90);

        $this->assertEquals('08:30', $clock->__toString());
    }

    public function testWrapAroundBackwards() {
        $clock = new Clock(0, 30);

        $clock = $clock->sub(60);

        $this->assertEquals('23:30', $clock->__toString());
    }

    public function testWrapAroundDay() {
        $clock = new Clock(5, 32);

        $clock = $clock->add(25 * 60);

        $this->assertEquals('06:32', $clock->__toString());
    }

    public function testWrapAroundDayBackwards() {
        $clock = new Clock(5, 32);

        $clock = $clock->sub(25 * 60);

        $this->assertEquals('04:32', $clock->__toString());
    }

    public function testEquivalentClocks() {
        $this->assertEquals(new Clock(15, 37), new Clock(15, 37));
    }

    public function testInequivalentClocks() {
        $this->assertNotEquals(new Clock(01, 01), new Clock(18, 32));
    }
}