>I got an error with "Cast from string "" to type 'Date' is not valid

The error means that the text in the field is not in the correct format. You may want to try something like below in order to trap for bad input...

        ' Variables
        Dim dInvDate As Date

        ' Trap Errors
        Try

            ' Attemp Convert
            dInvDate = CDate(txtInvoice.Text)

            ' Assign Date
            txtInvoice.Text = Format(dInvDate, "dd-mm-yyyy")

        Catch

            ' Alert User
            txtInvoice.Text = "Invalid Date"

        End Try

Hope that helps...