国际日期格式化失败

Question

I am trying with a child of DateTime to extend the functionality of DateTime for international formatting and simple database formatting. But it does not work with negative Date. var_dump of my Date object shows me the following:

object(Date)[98]
  public 'date' => string '-0001-11-30 00:00:00' (length=20)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/Paris' (length=12)

$this->format('Y-m-d') works fine, but not with IntlDateFormatter. This is my Code:

class Date extends DateTime
{

    public function __toString()
    {
        $fmt = new IntlDateFormatter(Locale::getDefault(), IntlDateFormatter::MEDIUM, IntlDateFormatter::MEDIUM);

        return $fmt->format($this);
    }

    public function toSqlDate()
    {
        return new String($this->format('Y-m-d H:i:s'));
    }
}

Edit: It´s new Date('0000-00-00') which causes the fail.