"add_header X-Upstream-Response-Time $upstream_response_time" not working

[Matt Albright]

I'm using nginx 0.5.35 (I know, we'll upgrade here soon) and have
$upstream_response_time in the access log, and it works great.

To track down a problem, I wanted to add a header to nginx's response that shows
the response time of the upstream server, so I tried:

add_header X-Upstream-Response-Time $upstream_response_time

Which seems like it would work, based on the docs for add_header, but I get:

X-Upstream-Response-Time: 1223488205.821

Which looks like a unix time to me, which isn't very useful.

Is there any way to get what I want?  I tried putting the add_header in http,
server, and location blocks, but all of them gave me a unix time.

Thanks for any help you can offer.

matt