public function click($time=null) {
        $this->total++;
        if ( ! $this->timestart ) {
            $this->timestart = (int) ( time() / $this->scale );
        }
        if ( ! $time ) $time = time();
        $time = (int) ($time / $this->scale);
        $delta = $time - $this->timestart;
        if ( $delta < 0 ) $delta = 0;  // 0 is OK
        if ( isset($this->buckets[$delta]) ) {
            $this->buckets[$delta]++;
        } else {
            $this->buckets[$delta] = 1;
        }
    }