If-Modified-Since

GET /jp/res/11107882.json HTTP/1.1
If-Modified-Since: Tue Jul 02 2013 08:41:51 GMT+0100 (IST)
Host: api.4chan.org
Connection: keep-alive

Why am I getting a 200 response code instead of 304 when I send this request? The headers I get back have last-modified: Mon, 01 Jul 2013 17:47:08 GMT so shouldn't it be returning a 304?

Some servers ignore it and the ones that don't interpret the header differently. I think 4chan uses Nginx or something, and Nginx in its default configuration apparently only sends 304 if the last modified dates match exactly.

http://trac.nginx.org/nginx/ticket/93