Make DateTime::createFromFormat() return child class instead of parent

This is the way to go. However, since what seems you want to do is to render the DateTime class extensible, I'd suggest you use static instead of self:

namespace NoiseLabs\DateTime;

class DateTime extends \DateTime
{
    static public function createFromFormat($format, $time)
    {
        $ext_dt = new static();
        $parent_dt = parent::createFromFormat($format, $time);

        if (!$parent_dt) {
            return false;
        }

        $ext_dt->setTimestamp($parent_dt->getTimestamp());
        return $ext_dt;
    }
}

It's not necessary if you don't plan on extending the class, but if someone ever does, it will prevent him from having to do the same workaround again.