Depends on how you're allowing them to log in. What script are you using (i.e. ASP, PHP, etc.) and are you using sessions?

If you are keeping the login with a persistent cookie, the easiest was is to set the cookie as a SESSION cookie. You do this by NOT setting an expiration date value in the cookie. When the browser is closed, the cookie dies and they must re-log in.

A session cookie WILL persist if a user does not close the browser - that is, if they go off the site, leave their browser open, then come back tomorrow they will still be logged in. If you want to avoid this, then you have to go back to setting an expire date and time in the cookie and figure out how to set it X hours ahead of the last activity.

The details of all this are too deep for a quick answer in this thread, search these forums for 'cookies' in the programming language the login scheme is using, you will find answers and examples.

A session cookie WILL persist if a user does not close the browser - that is, if they go off the site, leave their browser open, then come back tomorrow they will still be logged in.

True, but it depends on the scripting and how the sessions are defined.

For example, the default session timeout for VB (ASP) is 20 minutes and set automatically at the server level. Of course the coder has the option of increasing this timeout.

In this case, even if a user leaves their current browser open, their session will eventually timeout and they must re-login.

Code is in PHP...here is what i found in the header..what do i change?

}
function getexpirydate( nodays){
var UTCstring;
Today = new Date();
nomilli=Date.parse(Today);
Today.setTime(nomilli+nodays*24*60*60*1000);
UTCstring = Today.toUTCString();
return UTCstring;
}
function getcookie(cookiename) {
var cookiestring=""+document.cookie;
var index1=cookiestring.indexOf(cookiename);
if (index1==-1 ¦¦ cookiename=="") return "";
var index2=cookiestring.indexOf(';',index1);
if (index2==-1) index2=cookiestring.length;
return unescape(cookiestring.substring(index1+cookiename.length+1,index2));
}
function setcookie(name,value)
{
duration=30;

cookiestring=name+"="+escape(value)+";EXPIRES="+getexpirydate(duration);
document.cookie=cookiestring;

}
function delcookie(name)
{
cookiestring=name+"="+escape('')+";EXPIRES="+getexpirydate(-1);
document.cookie=cookiestring;

This code sets a cookie to expire at a certain date.

All you have to do is figure out how long you want the cookie to stay active.

If you would rather use sessions, Google the term: "session_start()"