private function updateTime()
    {
        // Date string generation is (relatively) expensive. Since we only need HTTP
        // dates at a granularity of one second we're better off to generate this
        // information once per second and cache it.
        $this->currentTime = \time();
        $this->currentHttpDate = \gmdate("D, d M Y H:i:s", $this->currentTime) . " GMT";

        foreach ($this->callbacks as $callback) {
            $callback($this->currentTime, $this->currentHttpDate);
        }
    }