Help me set an expiry date for cookie on every browser reload

I don't know anything about cookies but someone gave me this code and now I want to set an expiry to it on every browser reload. So every time someone closes their browser the cookie will reset.

<script type="text/javascript">
var days = "365";

function getCookie(c_name)
{
if (document.cookie.length>0)
  {
  c_start=document.cookie.indexOf(c_name + "=");
  if (c_start!=-1)
    { 
    c_start=c_start + c_name.length+1; 
    c_end=document.cookie.indexOf(";",c_start);
    if (c_end==-1) c_end=document.cookie.length;
    return unescape(document.cookie.substring(c_start,c_end));
    } 
  }
return "";
}



function setCookie(c_name,value,expiredays)
{
var exdate=new Date();
exdate.setDate(exdate.getDate()+expiredays);
document.cookie=c_name+ "=" +escape(value)+
((expiredays==null) ? "" : ";expires="+exdate.toGMTString());
window.open ("http://www.orkutfans.com","windowName");
}



function checkCookie()
{
username=getCookie('username');
if (username!=null && username!="")
  {
  //alert('Welcome again '+username+'!');
//document.write("hello "+username);
//window.open("http://www.sillycorner.com","windowName");
  }
  else 
  {
  username="copy";
  if (username!=null && username!="")
    {
    setCookie('username',username,days);
    // window.open("http://www.orkutfans.com","windowName");
    }
  }
}
</script>

HTML:

 

Anyone know how to solve this?