How to display the age of an nginx cached file in headers

The nginx documentation is quite exhaustive — there's no variable with the direct relative age of the cached file.

The best way would be to use the $upstream_http_ variable class to get the absolute age of the resource by picking up its Date header through $upsteam_http_date.

add_header X-Cache-Date $upstream_http_date;

For the semantic meaning of the Date header field in HTTP/1.1, refer to rfc7231#section-7.1.1.2, which describes it as the time of the HTTP response generation, so, basically, this should accomplish exactly what you want (especially if the backend runs with the same timecounter).

I spent some time attempting to solve this with the Nginx Perl module, which does not seem to have access to $upstream_http_NAME headers that would allow you to successfully calculate the current time from a timestamp header that your proxied application created during render time.

Alternatively, you could use a different caching layer architecture like Varnish Cache, which does indeed provide the Age HTTP response header:

http://book.varnish-software.com/3.0/HTTP.html#age

I made a solution that works for this, with the Lua module, in this question: Nginx: Add “Age” header, with Lua. Is this a good solution?

I'm going to post here the code, for any suggestion it would be better to discuss it in the other link, where I explain it in more detail.

map $upstream_http_Date $mapdate {    default $upstream_http_Date;     '' 'Sat, 21 Dec 2019 00:00:00 GMT';}

Inside location:

header_filter_by_lua_block {    ngx.header["Age"] = ngx.time() - ngx.parse_http_time(ngx.var.mapdate); }