Of course it can't be done on the server.

You can get it with JavaScript and pass it to the server in a number of ways if that's what you want.

Just off the top of my head, you could set a cookie with JavaScript when a page loads, and read it back in your cgi script. You could do it with Ajax, using an XMLHTTPRequest which talks to a script on your server in the background. You could hack a redirect so that foo.html redirects to bar.html?localtime=<their localtime>.

All of those are dependent on their having the right browser and JavaScript turned on, etc.