When the user unchecks the chkActive check box, the program prompts the user for a contract date. It checks whether the string is blank. It then uses the IsDate function to see if the value looks like a date. If it does look like a date, the program converts it to a date and compares it with the current date. If the date is later than today, the program prompts you for a new date and repeats the verification steps.

Private Sub chkActive_CheckedChanged(ByVal sender As _
    System.Object, ByVal e As System.EventArgs) Handles _
    chkActive.CheckedChanged
    'Check to see if there is a change in a Contract status
    If Me.txtAcctName.Text.Length > 0 Then

        ' If Current contract become invalid changes, 
        ' uncheck the "chkActive" box and validate date in
        ' input box
        If Me.chkActive.CheckState = CheckState.Unchecked _
            Then 'Checked = False Then
            Dim message, title As String
            Dim myValue As Object

            message = "Contract is no longer valid as of " & _
                "date entered:"   ' Set prompt.
            title = "Term Date"   ' Set the title.
            myValue = InputBox(message, title)

            If myValue Is "" Then
                ' If input box is empty, show message and
                ' set check box to True
                MessageBox.Show("No date was entered.")
                Me.chkActive.Checked = True
            Else
                ' If inputBox contains a value, then
                ' validate its contents:
                ' Set a flag to loop trough inputbox
                ' content until
                ' information is correct
                Dim checkDate As Boolean = False
                Do
                    Dim sPA As String = Trim(CStr(myValue))
                    '  Check to see if value entered is a
                    ' date
                    '  If not, then reset the inputBox 
                    If IsDate(sPA) = False Then
                        MessageBox.Show("Please enter a " & _
                            "valid date.")
                        checkDate = False
                        myValue = InputBox(message, title)
                    Else
                        '  If it is a date, make sure its
                        ' not later than today's date
                        If CDate(sPA) > Today Then
                            MessageBox.Show("Please enter a " & _
                                "valid date no later than " & _
                                "today.")
                            checkDate = False
                            myValue = InputBox(message, _
                                title)
                        Else
                            '  If all is well, procede 
                            Dim oPA As String = _
                                Me.txtAcctName.Text
                            Me.lblOldPvtAcct.Text = oPA
                            Me.lblTermDate.Text = _
                                CStr(myValue)

                            ' Set flag to true when
                            ' information entered into
                            ' inputbox is correct.
                            checkDate = True
                            'Reset text boxes for new entry
                            With txtAcctName
                                .Text = ""
                                .Enabled = True
                            End With

                            With chkActive
                                .Checked = False
                                .Enabled = False
                            End With
                        End If
                    End If
                Loop Until checkDate = True
            End If
        End If
    End If

End Sub