Why are you looking for difficult solutions like CGI or Javascript? When I want a Last modified date, I hit a in vi, followed by Last modified on Nov 27, 2002, followed by an escape. No need for the server to do work on each request, as with CGI or SSI solution (which often break any caching mechanisms), or to have to rely on whether the client has Javascript enabled.

It even works for plain text files.

Abigail