Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Me.MaskedTextBox1.Mask = "00/00/0000"
    Me.MaskedTextBox1.ValidatingType = GetType(System.DateTime)

    Me.ToolTip1.IsBalloon = True
End Sub

Private Sub MaskedTextBox1_TypeValidationCompleted(ByVal sender As Object, ByVal e As TypeValidationEventArgs) Handles MaskedTextBox1.TypeValidationCompleted
    If (Not e.IsValidInput) Then
        Me.ToolTip1.ToolTipTitle = "Invalid Date"
        Me.ToolTip1.Show("The data you supplied must be a valid date in the format mm/dd/yyyy.", Me.MaskedTextBox1, 0, -20, 5000)
    Else
        ' Now that the type has passed basic type validation, enforce more specific type rules.
        Dim UserDate As DateTime = CDate(e.ReturnValue)
        If (UserDate < DateTime.Now) Then
            Me.ToolTip1.ToolTipTitle = "Invalid Date"
            Me.ToolTip1.Show("The date in this field must be greater than today's date.", Me.MaskedTextBox1, 0, -20, 5000)
            e.Cancel = True
        End If
    End If
End Sub

' Hide the tooltip if the user starts typing again before the five-second display limit on the tooltip expires.
Private Sub MaskedTextBox1_KeyDown(ByVal sender As Object, ByVal e As KeyEventArgs) Handles MaskedTextBox1.KeyDown
    Me.ToolTip1.Hide(Me.MaskedTextBox1)
End Sub