New Post: MaskedTextBox for DateTime field doesn't work

I have created a MaskedTextBox inside my xaml window and bind a Datetime Property on my viewmodel.

I can't make it work because initial bind is missing seconds and milliseconds and when I try to edit value, can't be validated.

<xctk:MaskedTextBox Margin="10,0" MinWidth="200"  Mask="00/00/0000 00:00:00.000"
                                            Value="{Binding DataApertura, Mode=TwoWay}" 
                                            IsEnabled="{Binding ElementName=AperturaBtn, Path=IsChecked}"
                                            ValueDataType="{x:Type s:DateTime}"/>