public function toMJD($scale = null)
    {
        $mjd = static::MJD;

        if ($scale) {
            return bcsub(bcadd($this->jd, $this->dayFrac, $scale), $mjd,
              $scale);
        } else {
            return $this->jd + $this->dayFrac - $mjd;
        }
    }