The minimum caching time according to (Expires - Date)
	Headers Squid honors if the object can't be revalidated
	defaults to 60 seconds. In reverse proxy environments it
	might be desirable to honor shorter object lifetimes. It
	is most likely better to make your server return a
	meaningful Last-Modified header however. In ESI environments
	where page fragments often have short lifetimes, this will
	often be best set to 0.