class GdsApi::Response

def expires_in
  return unless headers[:date]
  max_age = Rack::Cache::CacheControl.new(headers[:cache_control])['max-age'] if headers[:cache_control]
  max_age ||= Time.parse(headers[:expires]) - Time.parse(headers[:date]) if headers[:expires]
  return unless max_age
  age = Time.now.utc - Time.parse(headers[:date])
  max_age.to_i - age.to_i
end