protected function keyForRange($key, \DateTime $start, \DateTime $end)
    {
        return 'range:' . $key . '_' . $this->dateString($start) . '_' . $this->dateString($end);
    }
protected function keyForRange($key, \DateTime $start, \DateTime $end)
    {
        return 'range:' . $key . '_' . $this->dateString($start) . '_' . $this->dateString($end);
    }
protected function keyForRange ( $ key , \ DateTime $ start , \ DateTime $ end ) { return 'range:' . $ key . '_' . $ this -> dateString ( $ start ) . '_' . $ this -> dateString ( $ end ) ; }
Generate key for key and date range.

@param string $key Counter key.
@param \DateTime $start Start date.
@param \DateTime $end End date.
@return string
Generate key for key and date range .