The following XAML can be applied to allow milliseconds to be edited when the Value of the xamMaskedEditor is databind to a DateTime type:

<igEditors:XamMaskedEditor
  x:Name="xamMaskedEditor1"
  Format="yyyy-MM-dd HH:mm:ss.fff"
  Value="{Binding Value}"
  ValueType="{x:Type sys:String}"/>