Response Header Last-Modified not updating

I'm having a caching issue with my site. I make a change to the js file and publish it to my development environment (unminified at the moment for testing). And I get the stale version of my file.

开发者_StackOverflow社区If I use the Page Speed plugin for Firebug I can see that inside the Response Header for that particular JS file the Last-Modified date says yesterday's date not today's datetime as I might expect.

Also just out of curiousity what is the "Date" field just after the "X-Powered-By"?

Thanks!

The Date header is the date and time that the response was sent (From Wikipedia).

I hate answering a question with a question, but is it possible you have a caching firewall between you and the web server?