DateTIme Serializable

/**
 * DateTimeWrapper
 *
 * This class exists because you cannot modify the microseconds of a native PHP 5.5
 * DateTime Object without creating a whole new instance.
 *
 * Additionally, this class allows you to  serialize Datetime Objects without losing 
 * the microseconds.
 *
 * @copyright 2014 Mshanken Communications
 * @author David Chan <dchan@mshanken.com>
 * @license BSD-3
 */
class DateTimeWrapper
{
    private $datetime = null;
    private $cold = null;

    public function __construct(DateTime $datetime)
    {  
        $this->datetime = $datetime;
    }

    public function __call($method, $params)
    {  
        return call_user_func_array(array($this->datetime, $method), $params);
    }

    public static function __callStatic($method, $params)
    {  
        return forward_static_call_array(array($this->datetime, $method), $params);
    }

    public function __sleep()
    {  
        $this->cold = $this->datetime->format('Y-m-d G:i:s.u');
        // unset($this->datetime);
        return array('cold');
    }
    
    public function __wakeup()
    {
        $this->__construct(DateTime::createFromFormat('Y-m-d G:i:s.u', $this->cold));
    }

    public function getDateTime()
    {  
        return $this->datetime;
    }
}

Is there any reason for using G instead of H for hours?