Expiring content mirrored using proxy_store?

[Ed W]

Hi, apart from cron, is there a good way to expire content mirrored 
using proxy_store?

My scenario is that I have a slow upstream server generating some 
javascript and I have set nginx to proxy this request and store it.  But 
it's only really reasonable to do so for about 10 mins

Phrased another way, I need a lightweight, easy to setup caching reverse 
proxy to accelerate a single dynamically generated javascript file.  
Anyone got any good recipes for that?

Ta

Ed W