<form id="form1" runat="server">
<div>
<asp:TextBox ID="Textbox1" runat="server"></asp:TextBox>
</div>
<script language="javascript" type="text/javascript">
if (document.getElementById("Textbox1").value == "") {
alert("Invalid date");
document.getElementById('Textbox1').value = '__/__/____';
document.getElementById("Textbox1").focus();
}
</script>
</form>
It is working properly for me.