public function elapsedTime() {
        $currentTime = microtime(true);
        if ($this->startTime === null) {
            $this->startTime            = $currentTime;
            $this->finishTime           = $currentTime;
        } else {
            $this->finishTime           = $currentTime;
            $this->elapsedTimeFromStart = $currentTime - $this->startTime;
        }
    }