I have a following servers structure:
content.server contains an Apache instance, which processes one simple PHP script. On the same node Nginx is set as a caching proxy to that Apache.
caching.server contains Nginx with proxy_cache directive, with upstream set to content.server

Php script generates an XML file, that has a lifetime of 30 minutes. This is achieved by setting headers in PHP code: "Cache-Control: max-age=1800", "Date: now()" and "Expires: now()+1800seconds".

content.server serves the cached file correctly:
> Date Wed, 06 Apr 2011 09:50:49 GMT
> Cache-Control public, must-revalidate, max-age=1800
> Expires Wed, 06 Apr 2011 10:14:25 GMT
> Etag 26ee7051c360ee28d2a5697fb2b97a03

But approximately half of the time caching.server serves the content where Date header is larger than Expires, and Expires is different from the one on content.server:
> Date Wed, 06 Apr 2011 09:49:58 GMT
> Cache-Control public, must-revalidate, max-age=1800
> Expires Wed, 06 Apr 2011 09:44:23 GMT
> Etag 26ee7051c360ee28d2a5697fb2b97a03


My questions is: what can cause such behaviour, when caching nginx sets its own Expires header?