VB.NET : รบกวนสอบถาม Error Conversion failed when converting date and/or time from character string

Imports System.Data
Imports System.Data.SqlClient
Partial Class FormEditAppointment
    Inherits System.Web.UI.Page
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack() Then
            Dim ConnString = "Server=localhost;Initial Catalog=SaleMarketing;Integrated Security=True"
            Dim Conn = New SqlConnection(ConnString)
            Conn.Open()


        
            Dim SQLstr = "SELECT * FROM APPOINTMENT where APPOINTMENT_ID='" & Request.QueryString("APPOINTMENT_ID") & "'"
            Dim com = New SqlCommand(SQLstr, Conn)
            Dim da = New SqlDataAdapter(com)
            Dim dt = New DataTable()
            da.Fill(dt)

            TextBox1.Text = dt.Rows(0)("APPOINTMENT_NAME")
            TextBox2.Text = dt.Rows(0)("APPOINTMENT_DESC")
            txtDate.Text = dt.Rows(0)("APPOINTMENT_START_TIME")
            txtDate1.Text = dt.Rows(0)("APPOINTMENT_NOTICE_DATE")
            DropDownList1.Text = dt.Rows(0)("APPOINTMENT_STATUS")
            DropDownList3.Text = dt.Rows(0)("APPOINTMENT_ASSIGN")
            
        End If
    End Sub
    Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim ConnString = "Server=localhost;Initial Catalog=SaleMarketing;Integrated Security=True"
        Dim Conn = New SqlConnection(ConnString)
        Conn.Open()
        Dim sqlstr = "update APPOINTMENT set APPOINTMENT_NAME='" & TextBox1.Text & "',APPOINTMENT_DESC='" & TextBox2.Text & "',APPOINTMENT_START_TIME='" & txtDate.Text & "',APPOINTMENT_NOTICE_DATE='" & txtDate1.Text & "',APPOINTMENT_STATUS='" & DropDownList1.Text & "',APPOINTMENT_ASSIGN='" & DropDownList3.Text & "'  where APPOINTMENT_ID='" & Request.QueryString("APPOINTMENT_ID") & "'"

        Dim com = New SqlCommand(sqlstr, Conn)
        com.ExecuteNonQuery()

        MsgBox("บันทึกข้อมูล 'เรียบร้อย'", MsgBoxStyle.Information, "เรียบร้อย")
        Response.Redirect("Appointment.aspx")

    End Sub
End Class