public WorkingTimeViewModel(WorkingTimeRange workingTimeRange)
        {
            DomainModel = workingTimeRange;

            TargetDate = new ReactivePropertySlim <DateTime>(DomainModel.TimePeriod.StartDateTime);

            StartTimeText = new ReactiveProperty <string>(DomainModel.TimePeriod.StartDateTime.ToString("HHmm"))
                            .SetValidateNotifyError(x => string.IsNullOrEmpty(x) ? "入力必須です" : null)
                            .SetValidateNotifyError(x => DateTimeParser.ConvertFromHHmm(x).HasValue == false ? "HHmm形式で入力してください" : null)
                            .AddTo(CompositeDisposable);

            EndTimeText = new ReactiveProperty <string>(DomainModel.TimePeriod.EndDateTime?.ToString("HHmm") ?? "")
                          .SetValidateNotifyError(x => string.IsNullOrEmpty(x) == false && DateTimeParser.ConvertFromHHmm(x).HasValue == false ? "HHmm形式で入力してください" : null)
                          .AddTo(CompositeDisposable);
        }