I will give you sample try as per your need
<script type="text/javascript">
function CompareDates() {
var str1 = document.getElementById('<%= txtDate.ClientID %>').value;
var str2 = document.getElementById('<%= txtDate1.ClientID %>').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("To date cannot be greater than from date");
return false;
}
}
</script>
<asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
<asp:TextBox ID="txtDate1" runat="server" onBlur="CompareDates();"></asp:TextBox>