It's interesting that Google Sites uses only one last modified timestamp per site. It also directly derives etag from it. This basically leads to situation where pages are reloaded from Google servers, even if nothing has changed. Expiry time is set to 5 seconds, which is ok. Because after that checking etag would tell if page has changed or not. But in this case, if any page of the site has been changed, etag is updated for all pages of the site. Etag content is quite clear, it looks to be last modification stamp with some extra data.This is totally working and nice solution AFAIK. Except the fact that this data is shared with all pages of the site.
Last-Modified: Sat, 23 Nov 2013 15:11:48 GMT
Expires: Sat, 23 Nov 2013 15:12:19 GMT
Date: Sat, 23 Nov 2013 15:12:14 GMT
Cache-Control: public, max-age=5