javscript in a gridview, How to comapare Two templetefield Value of Gridview

not working:::::::::
function CompareDates()
{

var grid = document.getElementById('<%= GrdvewTax.ClientID %>');
var str1,str2 ,txtRegdte, txtExpdte;
if (grid.rows.length > 0) {

for (var i = 1; i < grid.rows.length; i++) {

str1 = grid.rows[i].cells[3].getElementsByTagName("*")[0].value;
str2 = grid.rows[i].cells[4].getElementsByTagName("*")[0].value;

var str1 = document.getElementById("txtRegdte").value;
var str2 = document.getElementById("txtExpdte").value;

var dt1 = parseInt(str1.substring(0,2),10);
var mon1 = parseInt(str1.substring(3,5),10);
var yr1 = parseInt(str1.substring(6,10),10);
var dt2 = parseInt(str2.substring(0,2),10);
var mon2 = parseInt(str2.substring(3,5),10);
var yr2 = parseInt(str2.substring(6,10),10);
var date1 = new Date(yr1, mon1, dt1);
var date2 = new Date(yr2, mon2, dt2);

if(date2 < date1)
{
alert("Expdate cannot be greater than Regdate");
return false;
}
else
{
alert("Submitting ...");
document.form1.submit();
}

binding