how to solve ReferenceError: doStuff is not defined TypeError: document.getElementById(...) in javascript ?

function checkdate() {
           
            var dt = document.getElementById("TextBox1").value;


            var currdt = new Date();
            if (dt != null) {
                var myDate_array = dt.split("/");



                var enteredDate1 = new Date(myDate_array[2], parseInt(myDate_array[1]) - 1, parseInt(myDate_array[0]) + 2);


                var today1 = new Date();
                if (enteredDate1 < today1) {
                    document.getElementById("TextBox1").value = "dd/MM/yyyy";
                    alert("Case File date must be greater than current date");
                    return false;
                }

                var enteredDate = new Date(myDate_array[2], parseInt(myDate_array[1]) - 1, myDate_array[0]);
                var today = new Date();
                if (enteredDate > today) {
                    document.getElementById("TextBox1").value = "dd/MM/yyyy";
                    alert("Case File date must be less than current date");
                    return false;
                }

            }


        }

<asp:TextBox ID="TextBox1" runat="server" onfocus="if(this.value=='dd/mm/yyyy') this.value='';"
            onblur="if(this.value=='') this.value='dd/mm/yyyy';" Text="dd/mm/yyyy" onchange="javascript:checkdate();"></asp:TextBox>