public function humanize($duration = null)
    {
        if (null !== $duration) {
            $this->parse($duration);
        }

        if ($this->seconds > 0 || ($this->seconds === 0.0 && $this->minutes === 0 && $this->hours === 0 && $this->days === 0)) {
            $this->output .= $this->seconds . 's';
        }

        if ($this->minutes > 0) {
            $this->output = $this->minutes . 'm ' . $this->output;
        }

        if ($this->hours > 0) {
            $this->output = $this->hours . 'h ' . $this->output;
        }

        if ($this->days > 0) {
            $this->output = $this->days . 'd ' . $this->output;
        }

        return trim($this->output());
    }