class GdsApi::Response

def expires_in
  return unless headers[:date]
  age = Time.now.utc - Time.parse(headers[:date])
  if cache_control.max_age
    cache_control.max_age - age.to_i
  elsif headers[:expires]
    Time.parse(headers[:expires]).to_i - Time.now.utc.to_i
  end
end