JavaScript - Need Help Re. Javascript Dateadd In Forms

<script type="text/javascript"><!--
function updatesum() {
document.form.sum.value = (document.form.sum1.value -0) + (document.form.sum2.value -0);
}
//--></script>

<input type="button" value="D20" onclick="this.form.display.value = Math.round (20 * Math.random())" class="buttonHi" />&nbsp; <input name="display" type="text" size="6" value="" />

function validate_text(field,alerttext) {
	with(field) {
		
	  var re = /[a-zA-Z]/g;
  	  if(re.test(field)) { return true;}else{alert(alerttext);return false;}
	}
}
function validate_number(field,alerttext){
	  with(field) {
		  
	    var re = /[0-9]/g;
        if(re.test(field)) { return true;}else{alert(alerttext);return false;}
	  }
}



function validateForm(thisForm){
	with(thisForm)
	{
		if(validate_number(area,"wrong area code") == false) {
		
			area.focus();return false;	
		}
		if(validate_number(phone,"wrong phone") == false) {
		
			phone.focus();return false;	
		}
		if(validate_text(name_surname,"wrong name") == false) {
		
			name_surname.focus();return false;	
		}

	}
}

...<b>Name Surname :</b><input style="margin-left:10px;" type="text" name="name_surname"/>
<br /><br />
    <b>phone :</b><input type="text" style="margin-left:19px;" maxlength="4" size="4" name="area" /> 
-<input type="text" style="margin-left:5px;" maxlength="7" size="10" name="phone" /><br /><br />....

<input type="text" id="textInput" name="this.id">

<script type="text/javascript">
<!--
var lastMonth;
var thisMonth;

function diff(form)
	{
	var difference;
	difference = lastMonth - thisMonth;
	form.Fdiff.value = difference;
	}

	
	
function SetLast(usuage)
{
lastMonth = usuage.value;
}

function SetThis(usuage2)
{
thisMonth = usuage2.value;
}

	// -->
</script>

<center>
<form method="post">
<TABLE border=3>
<tr>
<tr>
<td><p>Last Months <br /> Usuage</p></td>
<td><p>This Months <br /> Usuage</p></td>
<td><p>Difference</p></td>
<TD><INPUT TYPE=BUTTON ONCLICK="diff(this.form)" VALUE="Calculate"></TD>

</tr>
<tr>
</tr>
<td><p><input type=text name=lastMonth size="4" onchange="SetLast(usuage)"></p></td>
<td><p><input type=text name=thisMonth size="4" onchange="SetThis(usuage2)"></p></td>
<td><p><INPUT TYPE=text NAME="Fdiff" VALUE="" SIZE="6"></p></TD>
<TD><p></p></tr>
</tr>
</tr>
</table>
</form>
</center>


</center>

	me.emailCheckout = function() {    

    itemsString = "";
    for( var current in me.items ){ 
        var item = me.items[current];
        itemsString += item.name + " " + item.quantity + " " + item.price + "\n";
    }   

    var form = document.createElement("form");
    simpleCart.empty();
    form.style.display = "none";
    form.method = "POST";
    form.action = "sendjs.php";
    form.acceptCharset = "utf-8";
    form.appendChild(me.createHiddenElement("jcitems", itemsString));
    form.appendChild(me.createHiddenElement("jctotal", me.total));
    document.body.appendChild(form);
    form.submit();
    document.body.removeChild(form);
}

<?php
    $to      = 'info@kheuning.nl';
    $subject = 'the subject';
    $jcitems = $_POST['jcitems'];
    $headers = 'From: webmaster@example.com' . "\r\n" .
               'Reply-To: webmaster@example.com' . "\r\n" .
               'X-Mailer: PHP/' . phpversion();
    mail($to, $subject, $jcitems, $headers);
    Header('Location: thankyou.html');
?>

function confirmSubmission()
	{
		var confirmed = validateData();
		return confirmed;
	}//confirmSubmission
	
	function validateData()
	{
		if (document.forms[0].elements[1].value == "")
		{
			alert("You must enter a first name.");
			document.forms[0].elements[1].focus();
			return false;
		}//blank first name
		
		if (document.forms[0].elements[2].value == "")
		{
			alert("You must enter a last name.");
			document.forms[0].elements[2].focus();
			return false;
		}//blank last name
		
		if (document.forms[0].elements[3].value == "")
		{
			alert("You must enter an address.");
			document.forms[0].elements[3].focus();
			return false;
		}//blank address
		
		if (document.forms[0].elements[4].value == "")
		{
			alert("You must enter a city.");
			document.forms[0].elements[4].focus();
			return false;
		}//blank city
		
		if (document.forms[0].elements[5].value == "")
		{
			alert("You must enter a state.");
			document.forms[0].elements[5].focus();
			return false;
		}//blank state
		
		if (document.forms[0].elements[6].value == "")
		{
			alert("You must enter a zipcode.");
			document.forms[0].elements[6].focus();
			return false;
		}
		else
		{
			if (/...../.test(document.forms[0].elements[6].value) == false)
			{
				alert("You must enter a valid zipcode.");
				document.forms[0].elements[6].focus();
				return false;
			}
		}//blank zip
		
		if (document.forms[0].elements[7].value == "")
		{
			alert("You must enter an e-mail address.");
			document.forms[0].elements[7].focus();
			return false;
		}
		else
		{
			if (/@/.test(document.forms[0].elements[7].value) == false || (document.forms[0].elements[7].value).indexOf(".", (document.forms[0].elements[7].value).indexOf("@")) == -1)
			{
				alert("You must enter a valid e-mail address.");
				document.forms[0].elements[7].focus();
				return false;
			}
		}//blank email
		
		if (document.forms[0].elements[8].value == "" || document.forms[0].elements[9].value == "" || document.forms[0].elements[10].value == "")
		{
			alert("You must enter a telephone number.");
			document.forms[0].elements[8].focus();
			return false;
		}
		else
		{
			if (/.../.test(document.forms[0].elements[8].value) == false)
			{
				alert("You must enter a valid area code.");
				document.forms[0].elements[8].focus;
				return false;
			}//area code
			else
				if (/.../.test(document.forms[0].elements[9].value) == false)
				{
					alert("You must enter a valid telephone exchange.");
					document.forms[0].elements[9].focus;
					return false;
				}//NNX
				else
					if (/..../.test(document.forms[0].elements[10].value) == false)
					{
						alert("Your number must be four characters.");
						document.forms[0].elements[10].focus;
						return false;
					}//number
		}//blank telephone
			
		setCookie('username', document.forms[0].elements[1].value, 2);
		return true;
	}//validateData
	
	function setCookie(c_name, value, expiredays)
	{
		var expDate=new Date();
		expDate.setDate(expDate.getDate() + expiredays);
		document.cookie=c_name + "=" + escape(value) + ((expiredays==null) ? "" : ";expires=" + expDate.toUTCString());

function checkCookie()
	{
		username = getCookie('username');
		if (username != null && username != "")
			alert('Welcome back, ' + username + '!');
	}//checkCookie
	
	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 "";
	}

myform = document.createElement("form");
	myform.method = "post";
	myform.action = "editdetails.php";
	myform.id = "editemail";
	myform.name = "editemail";
	
	var a=document.getElementById('editdetailstable').insertRow(2);
	var b=document.getElementById('editdetailstable').insertRow(3);
	
	var x=a.insertCell(0);
        x.innerHTML="New Email:  ";
	x.align = "right";

	var y=a.insertCell(1);
        y.innerHTML="<input type='text' name='email' id='email' />";
	
        var z=b.insertCell(0);
	z.innerHTML="<input type='button' value='Change' />"
	z.colSpan = "2";
	z.align = "center";

<body>
<script language="javascript">
//  database goes here
var username = new Array(3);
username[0]="mary";
username[1]="john";
username[2]="kevin";
var password = new Array(3);
password[0]="poppins";
password[1]="doe";
password[2]="poppins";

function Authenticate()
{
	// read the values from the form in the following variables
	var name = document.getElementById("username").value; 	var pass = document.getElementById("password").value; 
	var j;
	//check the username and password against our database
	for(i = 0; i < 3; i++)
	{
		if(name == username[i])
		{
			j = 1;
		}
		else if(pass == password[i])
		{
			j = 1;
		}
		else
		{
			j = 0;
		}			
		// output the appropriate message in the output Text Box.
	}
	if(j = 1)
	{
	document.getElementById("output").value = "correct login information";
	}
	else if(j = 0)
	{
	document.getElementById("output").value = "incorrect login information";
	}
}
</script>
<form name="form1">
 Username : <input name="username" type="text"><br><br>
 Password : <input name="password" type="password"><br><br>
 <input type="button" value="submit" onClick="Authenticate();"><br><br>
 
 Output   : <input name="output" type="text">
 
</form>

<input type="button" value="Next" onclick="submitForm(this.form);" id="button" />

<html>
<head>
Test



<SCRIPT LANGUAGE="JavaScript">
// JavaScript statements go here
function test(tmpForm) {
window.alert(tmpFrm.elements["weight"].value);
window.alert(tmpForm.elements["error"].innerHTML);
}
</SCRIPT>

</head>

<body>
<form id="form_137" name="form_137">
<input id="weight" name="weight" value="" />
<span id="error" name="error">abc</span>
<INPUT TYPE="button" id="btnSubmit" NAME="btnSubmit" VALUE="Submit" onClick="test(this.form);">

</form>

<form id="form_137_down" name="form_137_down">
<input id="weight" name="weight" value="" />
<span id="error" name="error">zzz</span>
<INPUT TYPE="button" id="btnSubmit" NAME="btnSubmit" VALUE="Submit" onClick="test(this.form);">

</form>

</body>
</html>