textbox 숫자를 날짜 형식으로 변경

     Dim Start_RD As String = TextBox15.Text

                Dim F_Start_RD As Date = Start_RD.Substring(0, 4) & "-" & Start_RD.Substring(4, 2) & "-" & Start_RD.Substring(6, 2)

                Label67.Text = F_Start_RD.ToString("yyyy-MM-dd")

                Dim End_RD As String = TextBox16.Text

                Dim F_End_RD As Date = End_RD.Substring(0, 4) & "-" & End_RD.Substring(4, 2) & "-" & End_RD.Substring(6, 2)

                Label68.Text = F_End_RD.ToString("yyyy-MM-dd")

                Dim 사용일 As Integer = DateDiff(DateInterval.Day, F_Start_RD, F_End_RD) + 1

                TextBox13.Text = 사용일