can anyone tweak my form cookie?

I got this script from Netmag the other day and I'd like to tweak it a bit but I can't seem to get it right.

The script warns users that they have already submitted a form when they click on submit, it gives the date/time of submission and gives then the option to re-submit.

And it goes like this:

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<FORM ACTION="http://www.netmag.co.uk" NAME="myform">
<INPUT TYPE=BUTTON VALUE="Submit This Form" onClick="mySubmit()">
</FORM>

</CENTER>

<SCRIPT>
var alreadyclicked = "no";
var cookiename = "datesubmitted";

function mySubmit(){
   if (alreadyclicked == "yes"){
      alert("Form already being processed.");
      return;
   }

   if (document.cookie.indexOf(cookiename) > -1){
      var mystart =document.cookie.indexOf(cookiename);
      var myend = document.cookie.indexOf(";", mystart);
      if (myend < 0){myend = document.cookie.length;}
      var mydate = document.cookie.substring(mystart +
cookiename.length + 1, myend+1);

      var myconfirm = "You submitted this form at ";
      myconfirm = myconfirm + mydate;
      myconfirm = myconfirm + ". Submit again?"
      ret = confirm(myconfirm);
      if (ret == false){
         return;
      }
   }

   mydate = new Date();
   document.cookie = cookiename + "=" + mydate;
   alreadyclicked = "yes";
   document.myform.submit();
}

</SCRIPT>

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I would like to tweak this script in two ways.

Firstly, I would like to REMOVE the option to re-submit,
and
secondly, I would like to set an expiry date of 3 months.

Any hot tips would be massively gratefully received.

Thanks.