Private Sub Command1_Click()
    If SetMonthView("12.04.1988", MonthView1) = False Then
        MsgBox "Fehler"
    End If
End Sub
 
Private Function SetMonthView(Datum As Date, View As MonthView) As Boolean
    With View
        .Year = Split(Datum, ".")(2)
        .Month = Split(Datum, ".")(1)
        .Day = Split(Datum, ".")(0)
    End With
    SetMonthView = True
    Exit Function
error:
End Function