class GdsApi::Response

def expires_at
  if headers[:date] && cache_control_headers = headers[:cache_control]
    max_age = Rack::Cache::CacheControl.new(cache_control_headers)['max-age']
    response_date = Time.parse(headers[:date])
    return response_date + max_age.to_i if max_age
  end
  Time.parse(headers[:expires]) if headers[:expires]
end