// One could do this with dates/time. But it's much simpler to use version numbers.
var current_version = 100;

// Is this the first time here?
var visited_before = get_cookie("last_visit");
if (visited_before)
{ var last_version = get_cookie("last_version");

var now = new Date();

// been here before --do some stuff
document.write("<CENTER><FONT COLOR=\"#aaaaFF\"> Thank you very much for returning.<br/>These pages ");

if (current_version != last_version)
{ document.write("<FONT COLOR=\"FF0000\">have changed</FONT>") }
else
{ document.write("are unchanged") }
document.write(" since your last visit.<br/></FONT></CENTER>");
}
else
{ document.write("<CENTER><FONT COLOR=\"#aaaaFF\"> Thank you very much for visiting.<br/></FONT></CENTER>");
}

// update the cookies; we need to set the expiration
var now = new Date();
var this_visit = now.getTime();
// Add a year to set the expiry

// The generic site cookie
set_cookie("welcomed", "hello");

now.setTime(now.getTime() + 365 * 24 * 60 * 60 * 1000);
set_cookie("last_visit", this_visit, now);
set_cookie("last_version", current_version, now);