Bruno Skvorc posted his solution to a typical job interview question on sitepoint. This is my attempt after 15min on paper and about 5mins of coding, commenting and (typo) debugging. PHP Job Interview Task: Day of Week Calculation http://www.sitepoint.com/php-job-interview-task-day-week-calculation/

PHP Job Interview Task: Day of Week Calculation (sitepoint)

<?php

class DateUtils
{
    public $offsetYear = 1900;
    public $daysInEvenMonth = 21;
    public $daysInOddMonth = 22;
    public $monthsInYear = 13;
    public $leapYearInterval = 5;
    public $daysInWeek = 7;

    public $weekDays = array(
        'Sunday',
        'Monday',
        'Tuesday',
        'Wednesday',
        'Thursday',
        'Friday',
        'Saturday'
    );

    /**
     * Prints the day of the week.
     *
     * Calculates the number of days since a known offset date
     * in order to determin the day of the week.
     * 
     * @param  int $year
     * @param  int $month
     * @param  int $day
     * @return string
     */
    public function dayOfTheWeek($year, $month, $day)
    {
        $yearsSinceOffsetYear = abs($this->offsetYear - $year);
        $leapYearsSinceOffsetYear = ceil($yearsSinceOffsetYear / $this->leapYearInterval);

        $oddMonthsThisYear = ceil(($month - 1) / 2);
        $evenMonthsThisYear = floor(($month - 1) / 2);

        $daysInEvenYear = ceil($this->monthsInYear / 2) * $this->daysInOddMonth
            + floor($this->monthsInYear / 2) * $this->daysInEvenMonth;

        $daysSinceOffsetYear =
            // how many days have passed in the full years
            $yearsSinceOffsetYear * $daysInEvenYear - $leapYearsSinceOffsetYear
            // how many days have passed in the full months this year
            + $oddMonthsThisYear * $this->daysInOddMonth + $evenMonthsThisYear * $this->daysInEvenMonth
            // days this month
            + $day;

        $dayOfTheWeekIndex = $daysSinceOffsetYear % $this->daysInWeek;
        return $this->weekDays[$dayOfTheWeekIndex];
    }
}

$dateUtils = new DateUtils();
printf(
    "Day Of The Week on 1900-01-01: %s\n".
    "Day Of The Week on 2013-11-17: %s\n",
    $dateUtils->dayOfTheWeek(1900, 1, 1),
    $dateUtils->dayOfTheWeek(2013, 11, 17)
);