Please try this HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
body
{
font-family: Arial;
font-size: 10pt;
}
</style>
<script type="text/javascript">
function TextBoxChange(text) {
var dropdownvalue = document.getElementById("<%=DropDownList1.ClientID %>").value;
switch (dropdownvalue.substring(0, 3)) {
case "Jan":
dropdownvalue = "01" + dropdownvalue.substring(6, 3);
break;
case "Feb":
dropdownvalue = "02" + dropdownvalue.substring(6, 3);
break;
case "Mar":
dropdownvalue = "03" + dropdownvalue.substring(6, 3);
break;
}
document.getElementById("<%=Textbox2.ClientID %>").value = text.value + " " + dropdownvalue;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox ID="Textbox1" runat="server" onchange="TextBoxChange(this);" />
<br />
<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem Text="Jan 10 END-1" />
<asp:ListItem Text="Feb 10 END-1" />
<asp:ListItem Text="Mar 10 END-1" />
</asp:DropDownList>
<br />
<asp:TextBox ID="Textbox2" runat="server" />
</div>
</form>
</body>
</html>
I have just add one extra parameter to get the number value
dropdownvalue = "01" + dropdownvalue.substring(6, 3);
