Calculate due dates, taking business days into account

FastForwarder.php

<?php

class FastForwarder
{
    protected $skipWhen = [];
    protected $numDays = 0;

    public static function createWithDays($num_days)
    {
        $ff = new static;
        $ff->numDays = $num_days;
        return $ff;
    }

    public function skipWhen($filter, $filter_name)
    {
        if (!is_bool(call_user_func($filter, new \DateTime()))) {
            throw new \InvalidArgumentException('$filter must accept a \DateTimeInterface and return a boolean.');    
        }
        
        $this->skipWhen[$filter_name] = $filter;
        return $this;
    }

    public function exec(\DateTimeInterface $dt, array &$filters_used = null)
    {
        $newDt = new \DateTime($dt->format('c'));
        $daysLeft = $this->numDays;
        $dateInterval = new \DateInterval('P1D');
        $filtersUsedMap = [];

        $increment = 1;
        while ($daysLeft > 0) {
            $skipFilterName = $this->getSkippedBy($newDt);
            if ($skipFilterName) {
                $filtersUsedMap[$skipFilterName] = true;
            } else {
                --$daysLeft;
            }

            $newDt->add($dateInterval);
        }

        do { // if end date falls on a skipped day, keep skipping until a valid day is found
            $skipFilterName = $this->getSkippedBy($newDt);
            if ($skipFilterName) {
                $filtersUsedMap[$skipFilterName] = true;
                $newDt->add($dateInterval);
            }
        } while ($skipFilterName);

        if ($filters_used !== null) {
            $filters_used = array_keys($filtersUsedMap);
        }

        return $newDt;
    }

    protected function getSkippedBy(\DateTime $dt)
    {
        foreach ($this->skipWhen as $name => $fn) {
            if (call_user_func($fn, $dt)) {
                return $name;
            }
        }

        return null;
    }
}

FastForwarderExample.php

<?php // load FastForwarder somehow...

$dayCount = 10;

$ff = FastForwarder::createWithDays($dayCount);
$ff->skipWhen(function (\DateTimeInterface $dt) {
    return in_array($dt->format('w'), [0, 6]);
}, 'weekend');
$ff->skipWhen(function (\DateTimeInterface $dt) {
    if ($dt->format('m') != 2) {
        return false;
    }

    return $dt->format('w') == 1 && $dt->format('d') > 14 && $dt->format('d') <= 21;
}, 'presidents_day');
$ff->skipWhen(function (\DateTimeInterface $dt) {
    if ($dt->format('m') != 11) {
        return false;
    }

    return $dt->format('w') == 4 && $dt->format('d') > 21 && $dt->format('d') <= 28;
}, 'thanksgiving');
$ff->skipWhen(function (\DateTimeInterface $dt) {
    return $dt->format('m') == 12 && $dt->format('d') == 25;
}, 'christmas');

$filtersUsed = [];

$startDate = new \DateTime('2015-11-20 09:00:00');
// nov 20 isn't skipped; reach nov 21 with 9 days remaining
// nov 21 is skipped; reached nov 22 with 9 days remaining
// nov 22 skipped; 9 days
// nov 23 not skipped; 8 days
// nov 24 not skipped; 7 days
// nov 25 not skipped; 6 days
// nov 26 skipped; 6 days
// nov 27 not skipped; 5 days
// nov 28 skipped; 5 days
// nov 29 skipped; 5 days
// nov 30 not skipped; 4 days
// dec 1 not skipped; 3 days
// dec 2 not skipped; 2 days
// dec 3 not skipped; 1 day
// dec 4 not skipped; reached dec 5 with 0 days remaining
// dec 5 is not a valid due date; advance to dec 6
// dec 6 is not a valid due date; advance to dec 7
// dec 7 is a valid due date; we're good to go!
$endDate = $ff->exec($startDate, $filtersUsed); // $filtersUsed is passed by ref and optional

echo $endDate->format('Y-m-d H:i:s') . "\n"; // 2015-12-07 09:00:00
echo implode(', ', $filtersUsed) . "\n"; // weekend, thanksgiving

$otherStartDate = new \DateTime('2015-02-12 09:00:00');
$otherEndDate = $ff->exec($otherStartDate, $filtersUsed);

echo $otherEndDate->format('Y-m-d H:i:s') . "\n"; // 2015-02-27 09:00:00
echo implode(', ', $filtersUsed) . "\n"; // weekend, presidents_day