public function setTimeDependent($timeDependent)
    {
        if (gettype($timeDependent) === 'boolean') {
            $this->timeDependent = $timeDependent;
        } else {
            $msg = "The timeDependent argument must be a boolean value, '" . gettype($timeDependent) . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }