When pressing Enter or Tab in the new row the ParsingDateTime event is fired after the editor is validated. Use attached to reproduce.

 

private void GridView_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    var editor = gridView.ActiveEditor as RadDateTimeEditor;
    var editorElement = editor.EditorElement as RadDateTimeEditorElement;
    editorElement.TextBoxElement.MaskType = MaskType.FreeFormDateTime;
    var provider = ((FreeFormDateTimeProvider)editorElement.TextBoxElement.Provider);
    editorElement.TextBoxElement.Provider = new MyFreeFormDateTimeProvider(provider.Mask, provider.Culture, provider.Owner);
}

class MyFreeFormDateTimeProvider : FreeFormDateTimeProvider
{
    public MyFreeFormDateTimeProvider(string mask, CultureInfo culture, RadMaskedEditBoxElement owner) : base(mask, culture, owner)
    {
    }
    public override bool Validate(string stringValue)
    {
        //do your code here
        return base.Validate(stringValue);
    }
}