fastcgi_cache and 304 response

[wishmaster]

Hi,

I use nginx + php-fpm (via fcgi) and needed responses from php-server are putting into cache. I have one thought, could be better send cached pages to clients from cache with 304 code instead 200.
 So we must know time when response has been cached (something like variable) and send 304 response as long as page will be in cache (this time we know).
 
 Reading source codes I have not find any appropriate variable.
 
 Any ideas?