public function hasFor($key, \DateTime $date)
    {
        return array_key_exists($this->keyFor($key, $date), $this->data);
    }
public function hasFor($key, \DateTime $date)
    {
        return array_key_exists($this->keyFor($key, $date), $this->data);
    }
public function hasFor ( $ key , \ DateTime $ date ) { return array_key_exists ( $ this -> keyFor ( $ key , $ date ) , $ this -> data ) ; }
Determine if value exists for a given date.

@param string $key Counter key.
@param \DateTime $date Date.
@return bool
Determine if value exists for a given date .