SO - BusinessHoursCalc

<?php
class BusinessCalc
{
    const BUSINESS_START = 9;
    const BUSINESS_END = 17;
    const SECONDS_PER_HOUR = 3600;
    const SECONDS_PER_DAY = 86400;

    /**
     * @var array $_excludedDays
     */
    protected $_excludedDays = array(6, 7);

    /**
     * @var DateTime $_start
     */
    protected $_start;

    /**
     * @param DateTime $start
     */
    public function __construct($start)
    {
        $this->_start = $start;
    }

    /**
     * @param DateTime $now
     * @return integer
     */
    public function calculateSeconds(DateTime $now)
    {
        $difference = 0;
        if ($this->_isBusinessDay($this->_start)
            && $this->_isInBusinessTime($this->_start)
        ) {
            $difference += $this->_getEndOfBusinessDay($this->_start)->getTimestamp()
                - $this->_start->getTimestamp();
        }

        if ($this->_isBusinessDay($now)
            && $this->_isInBusinessTime($now)
        ) {
            $difference += $this->_getStartOfBusinessDay($now)->getTimestamp()
                - $now->getTimestamp();
        }

        // the following part looks crappy but works ;)
        // adds business hours to difference
        // checks if the last day is reached, skips this, cause we already have this above
        $dayIterator = $this->_start->getTimestamp();
        while ($dayIterator + self::SECONDS_PER_DAY < $now->getTimestamp()
            && $this->_getEndOfBusinessDay(new DateTime('@' . $dayIterator))->getTimestamp() < $now->getTimestamp()
        ) {
            $dayIterator += self::SECONDS_PER_DAY;
            if ($this->_isBusinessDay(new DateTime('@' . $dayIterator))) {
                $difference += (self::BUSINESS_END - self::BUSINESS_START) * self::SECONDS_PER_HOUR;
            }

        }

        return $difference;
    }

    /**
     * @param DateTime $time
     * @return DateTime
     */
    protected function _getEndOfBusinessDay($time)
    {
        $endOfDay = new DateTime('@' . $time->getTimestamp());
        $endOfDay->setTime(self::BUSINESS_END, 0, 0);
        return $endOfDay;
    }

    /**
     * @param DateTime $time
     * @return DateTime
     */
    protected function _getStartOfBusinessDay($time)
    {
        $startOfDay = new DateTime('@' . $time->getTimestamp());
        $startOfDay->setTime(self::BUSINESS_START, 0, 0);
        return $startOfDay;
    }

    /**
     * @param DateTime $time
     * @return integer
     */
    protected function _getWeekDay($time)
    {
        return date('N', $time->getTimestamp());
    }

    /**
     * @param DateTime $time
     * @return boolean
     */
    protected function _isInBusinessTime(DateTime $time)
    {
        if ($time->getTimestamp() <= $this->_getEndOfBusinessDay($time)->getTimestamp()
            && $time->getTimestamp() >= $this->_getStartOfBusinessDay($time)->getTimestamp()
        ) {
            return true;
        } else {
            return false;
        }
    }

    /**
     * @param DateTime $time
     * @return boolean
     */
    protected function _isBusinessDay($time)
    {
        $weekday = date('N', $time->getTimestamp());
        if (in_array($weekday, $this->_excludedDays) === true) {
            return true;
        } else {
            return false;
        }
    }
}


error_reporting(-1);
$start = time() - 12234567;
$a = new BusinessCalc(new DateTime('@' . $start));
echo $a->calculateSeconds(new DateTime());