public function verifySettings()
    {
        if ($this->dateTime === null && $this->seconds < 0) {
            $this->setError(
                "You must provide a datetime or the number of "
                . "seconds."
            );
        } elseif ($this->seconds < 0) {
            $this->setError(
                "Date of " . $this->dateTime
                . " results in negative seconds"
                . " value relative to epoch (January 1, 1970, 00:00:00 GMT)."
            );
        }
    }