Setting Date Format

Setting Date Format in C#.

I am trying to set date format as following

1. Whenever user enters 2 numbers automatically one sign like / is created
2. Whenever user enters another 2 numbers automatically one sign like / is created again

<script language="javascript" type ="text/javascript">
        function formatDate(obj) {
            //04/04/2011 PP
            var v_date = obj.value
            var numCount = v_date.length;
            if ((event.keyCode >= 48 && event.keyCode <= 57)) {
                if (numCount == 2) {
                    obj.value = v_date + "/"
                }
                else if (numCount == 5) {
                    obj.value = v_date + "/"
                }
            }
            else {
                event.keyCode = 0;
            }

        }         
    </script>  

<input type="text" id="txtfrmdt"  onkeypress="javascript:formatDate(this);" />