protected override bool Filter(IMappedFeature mappedFeature)
        {
            var  recording = mappedFeature as Recording;
            bool result    = (recording != null);

            if (result)
            {
                DateTime?recordedAt = recording.RecordedAt;
                result = (recordedAt != null);

                if (result)
                {
                    var dateTime = (DateTime)recordedAt;
                    int year     = dateTime.Year;
                    int month    = dateTime.Month;

                    if (!YearMonth.ContainsKey(year))
                    {
                        YearMonth.Add(year, month);
                        ChangeHistoricalDate();
                    }
                    else
                    {
                        YearMonth[year] = month;
                    }
                }
            }

            return(result);
        }