Expires is a Http header that allows you to define when a resource (image, css, ...) will need to be reloaded. It is a String representation of a Date in the format EEE, dd MMM yyyy HH:mm:ss z. There is lots of Java code available on the internet to produce the Expires header, the most part is similar to the following code I used during my experiments:

 	public static void setCacheExpireDate(HttpServletResponse response,
			int seconds) {
		if (response != null) {
			Calendar cal = new GregorianCalendar();
			cal.roll(Calendar.SECOND, seconds);
			response.setHeader("Cache-Control", "PUBLIC, max-age=" + seconds + ", must-revalidate");
			response.setHeader("Expires", htmlExpiresDateFormat().format(cal.getTime()));
		}
	}

        public static DateFormat htmlExpiresDateFormat() {
            DateFormat httpDateFormat = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss z", Locale.US);
            httpDateFormat.setTimeZone(TimeZone.getTimeZone("GMT"));
            return httpDateFormat;
        }

With the code, the browser started receiving headers like these:

        Last-Modified:  Sun, 19 Sep 2004 22:06:49 GMT
        Cache-Control:  PUBLIC, max-age=57600, must-revalidate
        Expires:         Thu, 09 Aug 2007 05:22:55 GMT
        Content-Type:   image/gif
        Content-Length: 377
        Date:             Wed, 08 Aug 2007 13:22:55 GMT
        Server:           Apache-Coyote/1.1

It was supposed to be a simple configuration step of my website optimization, but - after trying different values for the headers, including Expires header, Cache-Control and Last-Modified - the YSlow was still saying the Expires header was not present. I got confused and started to navigate on discussions lists and blogs when I finally figured out the problem: YSlow uses the idea of a far future.

A far future Expires header

Well, far future seems to be one of that relatives measures, when for a butterfly it could be 23 hours and for a mountain it could be one million years. So, I suppose the far notion should be pre-defined some way. I didn't find and I confess didn't search too much about the standard values adopted by YSlow, but from my desktop experiences I found it: The YSlow magic number far future is

That's it, everything more than two days seems to be far enough to YSlow - a minor detail in this complex task of websites tunning. And it is not an official information, just an empirical observation. If you need more precise guidance about YSlow, the Chief Performance Yahoo!, Steve Souders, is launching a book entirely dedicated to website optimization - a promising publishing from a company that serves millions of users every day :)

A last and very important advise: don't take decisions based only on YSlow. It is just an indicator, a tool created for Yahoo site optimization that can give you a glimpse about the quality of your web site - a helpful tool that should evolve from now on to become part of the arsenal of tips and tricks for websites tunning.