Crude attempt at supporting Elasticsearch date math values in PHP's DateTime object

DateTime.php

<?php

namespace MyNamespace;

/**
 * @author Ton Sharp
 * @author Brian Litzinger
 * @see    https://gist.github.com/66Ton99/60571ee49bf1906aaa1c
 * @see    https://www.elastic.co/guide/en/elasticsearch/reference/5.3/common-options.html#date-math
 */
class DateTime extends \DateTime
{
    /**
     * @param int|null $year
     * @param int|null $month
     * @param int|null $day
     *
     * @return $this
     */
    public function setDate($year, $month, $day)
    {
        if (null == $year) {
            $year = $this->format('Y');
        }

        if (null == $month) {
            $month = $this->format('n');
        }

        if (null == $day) {
            $day = $this->format('j');
        }

        $daysInMonth = cal_days_in_month(CAL_GREGORIAN, $month, $year);
        $day = $day > $daysInMonth ? $daysInMonth : $day;
        $return = parent::setDate($year, $month, $day);

        return $return;
    }
    /**
     * @inheritdoc
     */
    public function modify($modify)
    {
        $pattern = '/(now)?([-+]\d+\w)(\/\w)?/';

        preg_match_all($pattern, $modify, $allMatches, PREG_SET_ORDER);

        if (empty($allMatches)) {
            return $this;
        }

        foreach ($allMatches as $matches) {
            $modify = preg_replace_callback(
                $pattern,
                function ($matches) use ($pattern) {
                    if (empty($matches[0])) {
                        return;
                    }

                    $value = $this->stringify($matches[2]);
                    $modifier = $matches[3] ?? null;

                    $value = parent::modify($value);

                    if ($value && $modifier) {
                        $this->applyModifier($modifier);
                    }

                    return;
                },
                $modify
            );

            if ($modify = trim($modify)) {
                return parent::modify($modify);
            }
        }

        return $this;
    }

    private function stringify($value)
    {
        $pattern = '/(\d+)(\w+)/';

        return preg_replace_callback(
            $pattern,
            function ($matches) {
                if (empty($matches)) {
                    return '';
                }

                return $matches[1].$this->transpose($matches[2]);
            },
            $value
        );
    }

    private function transpose($value)
    {
        $options = [
            'd' => ' day',
            'h' => ' hour',
            'H' => ' hour',
            'm' => ' minutes',
            'M' => ' month',
            's' => ' second',
            'w' => ' week',
            'y' => ' year',
        ];

        return $options[$value] ?? '';
    }

    private function applyModifier($modifier)
    {
        switch ($modifier) {
            case '/d':
                parent::modify('today midnight');
                break;
            case '/M':
                parent::modify('first day of this month');
                parent::modify('today midnight');
                break;
        }
    }
}