Synopsis: Javascript command lastmodified not working State-Changed-From-To: open-closed State-Changed-By: marc State-Changed-When: Mon Jul 28 14:22:17 PDT 1997 State-Changed-Why: The client is the one that does that, not the server. Apache has nothing to do with how the client figures it out. What you are probably trying to do is get the last modified date from a page that doesn't have one, like a page parsed for SSI. As is documented, last-modified dates aren't sent with SSI documents because they change dynamically.
If you don't think this is what is happening, someone may be able to comment futher but you need to give an actual URL to a page that shows the problem.
