Part of a code challenge for a job application in 2007 was to create a pay date calculator. Presented here is the exact code created for it over 6 years ago. Needless to say some of my coding practices and styles have further developed since that time, such as placing the trailing parenthesis on a separate line, using lowerCamelCase for variable…

paydate_calculator.class

<?php

class Paydate_Calculator {
	/**
	 * Indicate if we're going forward (1) or backward (-1) when we make our adjustments.
	 * @var int
	 */
	private $due_date_adjustment = 1;

	/**
	 * Keep track of how many pay days we've skipped ahead
	 * @var int
	 */
	private $pay_day_adjustment = 0;

	/**
	 * Keep track of the fund day given to us.
	 * @var unix_timestamp
	 */
	private $fund_day;

	/**
	 * Array of unix timestamp holidays
	 * @var array
	 */
	private $holiday_array;

	/**
	 * String specifying interval between pay periods
	 * @var string
	 */
	private $pay_span;

	/**
	 * Keep track of the pay day given to us.
	 * @var unix_timestamp
	 */
	private $pay_day;

	/**
	 * Boolean value specifying if they have direct deposit
	 * @var boolean
	 */
	private $direct_deposit;

	/**
	 * This function determines the first available due date following the funding of a loan.
	 * The paydate will be at least 10 days in the future from the $fund_day. The due_date will 
	 * fall on a day that is a paydate based on their paydate model specified by '$pay_span' 
	 * unless the date must be adjusted forward to miss a weekend or backward to miss a holiday.
	 * Holiday adjustment takes precedence over Weekend.
	 *
	 * @param unix_timestamp $fund_day The day the loan was funded.
	 * @param array $holiday_array An array of unix timestamp's containing holidays.
	 * @param string $pay_span A string representing the frequency at which the customer is paid.
	 * (weekly,bi-weekly,monthly)
	 * @param unix_timestamp $pay_day A timestamp containing one of the customers paydays
	 * @param bool $direct_deposit A boolean determining whether or not the customer receives 
	 * their paycheck via direct deposit.
	 * @return unix_timestamp A unix timestamp representing the determined due date.
	 */
	public function Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
			$direct_deposit) {
		/* Make sure $pay_span is an expected frequency */
		if (!in_array($pay_span, array('weekly', 'bi-weekly', 'monthly'))) {
			trigger_error('Unexpected pay span [' . htmlspecialchars($pay_span) . '] specified', 
				E_USER_ERROR);
			return 0;
		}

		/* Set/Refresh all of our initial values. Also standardize dates for easier comparison */
		$this->fund_day = $this->getDayUnixTimestamp($fund_day);
		$this->holiday_array = array_map(array(&$this, 'getDayUnixTimestamp'), $holiday_array);
		$this->pay_span = $pay_span;
		$this->pay_day = $this->getDayUnixTimestamp($pay_day);
		$this->direct_deposit = (bool)$direct_deposit;
		$this->pay_day_adjustment = 0;
		$this->due_date_adjustment = 1;

		/*
		 * Get first pay date after fund date. This will be our first due date.
		 * We could optimize this by getting the first pay date 10 days after fund day, but
		 * in one very specific circumstance that could lead to the due date being a whole pay date
		 * after the 10 days past the fund date, even though, due to the weekend plus, it would have 
		 * been right after the 10 day fund date.
		 */
		$next_pay_day = $this->pay_day;
		while ($next_pay_day <= $this->fund_day) {
			$next_pay_day = $this->getNextPayDay();
		}
		$due_date = $next_pay_day;

		$minimum_due_date = $this->getDayAdjustment($this->fund_day, 10);
		do {
			/* Add a day if this isn't a direct deposit */
			if (!$this->direct_deposit) {
				$due_date = $this->getDayAdjustment($due_date, 1);
			}

			/* Check for weekends and holidays and adjust as necessary */
			$this->due_date_adjustment = 1;
			while (!$this->isAcceptableDate($due_date)) {
				$due_date = $this->getDayAdjustment($due_date, $this->due_date_adjustment);
			}
			
			/* If we're less than fund day + 10 days, also get the next pay day and then loop */
		} while ($due_date < $minimum_due_date && $due_date = $this->getNextPayDay());

		return $due_date;
	}

	/**
	 * Take a given unix timestamp and return one that is at the start of the day.
	 * This allows us to standize all timestamps for easier comparison, since we only care about
	 * the specific year/day and not the hour/seconds.
	 *
	 * @param unix_timestamp $unix_timestamp Timestamp to standardize.
	 * @return unix_timestamp Timestamp of the beginning of the input timestamp day.
	 */
	private function getDayUnixTimestamp($unix_timestamp) {
		return mktime(0, 0, 0,
			date('m', $unix_timestamp),
			date('d', $unix_timestamp),
			date('Y', $unix_timestamp),
			false);
	}

	/**
	 * Check if a timestamp given is during the weekend or holiday.
	 * If it's during a holiday then reverse the due date adjustment.
	 *
	 * @param unix_timestamp $due_date Unix timestamp of day to check against.
	 * @return boolean True if it is acceptable, false otherwise.
	 */
	private function isAcceptableDate($due_date) {
		if ($this->isTimestampInWeekend($due_date)) {
			return false;
		} else if ($this->isTimestampInHoliday($due_date)) {
			$this->due_date_adjustment = -1;
			return false;
		}

		return true;
	}

	/**
	 * Check to see if the timestamp is a weekend day.
	 *
	 * @param unix_timestamp $unix_timestamp The timestamp to check the day of.
	 * @return boolean True if a weekend day, false otherwise.
	 */
	private function isTimestampInWeekend($unix_timestamp) {
		/* Sunday = 0, Saturday = 6 */
		$weekend = array(0, 6);
		$timestamp_day = date('w', $unix_timestamp);

		return in_array($timestamp_day, $weekend);
	}

	/**
	 * Check if a given unix timestamp is in the holiday array.
	 *
	 * @param unix_timestamp $unix_timestamp The timestamp to check against holidays.
	 * @return boolean True if a holiday, false otherwise.
	 */
	private function isTimestampInHoliday($unix_timestamp) {
		return in_array($unix_timestamp, $this->holiday_array);
	}

	/**
	 * Function returns the unix timestamp of the next day based on unix timestamp input.
	 *
	 * @param unix_timestamp $unix_timestamp A timestamp to adjust the days of
	 * @param integer $adjustment How many days to adjust the timestamp by
	 * @return unix_timestamp A unix timestamp of the adjusted time
	 */
	private function getDayAdjustment($unix_timestamp, $adjustment) {
		return mktime(0, 0, 0, 
			date('m', $unix_timestamp), 
			date('d', $unix_timestamp) + $adjustment, 
			date('Y', $unix_timestamp),
			false);
	}

	/**
	 * Calculate the next pay day based on the pay span.
	 * Special care must be taken for monthly pay spans because not all months will have 
	 * the same day (e.g. February doesn't have a 30th). When this occurs we set the pay day as 
	 * the last day of the month instead.
	 *
	 * @return unix_timestamp The next pay day.
	 */
	private function getNextPayDay() {
		$this->pay_day_adjustment++;

		if ($this->pay_span == 'monthly') {
			$pay_day = mktime(0, 0, 0,
				date('m', $this->pay_day) + $this->pay_day_adjustment,
				date('d', $this->pay_day),
				date('Y', $this->pay_day),
				false);

			/* If the month actually jumps two months ahead go to the last day of the month. */
			if ((date('m', $this->pay_day) + $this->pay_day_adjustment) - date('m', $pay_day) < 0) {
				$pay_day = mktime(0, 0, 0,
					date('m', $this->pay_day) + $this->pay_day_adjustment + 1,
					0,
					date('Y', $this->pay_day),
					false);
			}
		} else {
			$adjustment_rate = ($this->pay_span == 'weekly') ? 7 : 14;
			$adjustment = $this->pay_day_adjustment * $adjustment_rate;
			$pay_day = $this->getDayAdjustment($this->pay_day, $adjustment);
		}

		return $pay_day;
	}
}

?>

scenario_tests.php

<?php

require_once('paydate_calculator.class');

/* Federal Holiday List for 2007 copied from: http://www.opm.gov/fedhol/2007.asp */
$holiday_array = array(
	mktime(0, 0, 0, 1, 1, 2007, false), /* New Year's Day */
	mktime(0, 0, 0, 1, 15, 2007, false), /* Birthday of Martin Luther King, Jr */
	mktime(0, 0, 0, 2, 19, 2007, false), /* Washington's Birthday */
	mktime(0, 0, 0, 5, 28, 2007, false), /* Memorial Day */
	mktime(0, 0, 0, 7, 4, 2007, false), /* Independence Day */
	mktime(0, 0, 0, 9, 3, 2007, false), /* Labor Day */
	mktime(0, 0, 0, 10, 8, 2007, false), /* Columbus Day */
	mktime(0, 0, 0, 11, 12, 2007, false), /* Veterans Day */
	mktime(0, 0, 0, 11, 22, 2007, false), /* Thanksgiving Day */
	mktime(0, 0, 0, 12, 25, 2007, false)); /* Christmas Day */

$paydate_calculator = new Paydate_Calculator;

/*
 * Scenario 1:
 * Monthly pay span makes next pay day fall on a non-existant day of the next month.
 * Expected result: next due date is the last day of the that month.
 */
$fund_day = mktime(0, 0, 0, 2, 5, 2007, false);
$pay_day = mktime(0, 0, 0, 1, 30, 2007, false);
$pay_span = 'monthly';
$direct_deposit = true;

$due_date = $paydate_calculator->Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
	$direct_deposit);
if ($due_date == mktime(0, 0, 0, 2, 28, 2007, false)) {
	echo 'Scenario 1: Passed.' . "\n";
} else {
	echo 'Scenario 1: Failed!' . "\n";
}

/*
 * Scenario 2:
 * bi-weekly pay span makes the next pay date fall 9 days after due date but on Saturday.
 * Expected result: next due date will be that Monday, 11 days after due date.
 */
$fund_day = mktime(0, 0, 0, 2, 15, 2007, false);
$pay_day = mktime(0, 0, 0, 2, 10, 2007, false);
$pay_span = 'bi-weekly';
$direct_deposit = true;

$due_date = $paydate_calculator->Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
	$direct_deposit);
if ($due_date == mktime(0, 0, 0, 2, 26, 2007, false)) {
	echo 'Scenario 2: Passed.' . "\n";
} else {
	echo 'Scenario 2: Failed!' . "\n";
}

/*
 * Scenario 3:
 * The due date falls on a weekend, but the following Monday is a holiday.
 * Expected result: The next due date will be the friday before the weekend.
 */
$fund_day = mktime(0, 0, 0, 2, 5, 2007, false);
$pay_day = mktime(0, 0, 0, 2, 3, 2007, false);
$pay_span = 'bi-weekly';
$direct_deposit = true;

$due_date = $paydate_calculator->Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
	$direct_deposit);
if ($due_date == mktime(0, 0, 0, 2, 16, 2007, false)) {
	echo 'Scenario 3: Passed.' . "\n";
} else {
	echo 'Scenario 3: Failed!' . "\n";
}

/*
 * Scenario 4:
 * The due date falls on a holiday, but the previous day is less than 10 days after the fund date.
 * Expected result: The due date will jump to the next pay date, a week later.
 */
$fund_day = mktime(0, 0, 0, 6, 24, 2007, false);
$pay_day = mktime(0, 0, 0, 6, 27, 2007, false);
$pay_span = 'weekly';
$direct_deposit = true;

$due_date = $paydate_calculator->Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
	$direct_deposit);
if ($due_date == mktime(0, 0, 0, 7, 11, 2007, false)) {
	echo 'Scenario 4: Passed.' . "\n";
} else {
	echo 'Scenario 4: Failed!' . "\n";
}

/*
 * Scenario 5:
 * Direct deposit is off.
 * Expected result: The due date will be the day after the next pay date.
 */
$fund_day = mktime(0, 0, 0, 3, 1, 2007, false);
$pay_day = mktime(0, 0, 0, 3, 1, 2007, false);
$pay_span = 'weekly';
$direct_deposit = false;

$due_date = $paydate_calculator->Calculate_Due_Date($fund_day, $holiday_array, $pay_span, $pay_day, 
	$direct_deposit);
if ($due_date == mktime(0, 0, 0, 3, 16, 2007, false)) {
	echo 'Scenario 5: Passed.' . "\n";
} else {
	echo 'Scenario 5: Failed!' . "\n";
}

?>