code: function (args) {
                  args.HasError = false;
                  args.ErrorMessage = "";
  
                  //no dates - > nothing to validate
                  if (!_.isDate(this.From) || !_.isDate(this.To))
                      return;
  
                  if (self.FromDatePart.isAfter(self.ToDatePart)) {
                      args.HasError = true;
                      args.ErrorMessage = customErrorMessage({ Msg: "Date from '{From}' must be before date to '{To}'.", Format: 'MM/DD/YYYY' }, this);
                      args.TranslateArgs = { TranslateId: 'BeforeDate', MessageArgs: this, CustomMessage: customErrorMessage };
                      return;
                  }
  
                  var minDays = 1;
                  var maxDays = 25;
  
                  //maximal duration
                  if (self.IsOverLimitRange || (self.VacationDaysCount > maxDays || self.VacationDaysCount < minDays)) {
                      args.HasError = true;
                      var messageArgs = { MaxDays: maxDays, MinDays: minDays };
                      args.ErrorMessage = Utils.StringFce.format("Vacation duration value must be between {MinDays] and {MaxDays} days.", messageArgs);
                      args.TranslateArgs = { TranslateId: 'RangeDuration', MessageArgs: messageArgs };
                      return;
                  }
  
                  var diff = _.difference(self.ExcludedDaysDatePart, self.RangeDays);
                  if (diff.length != 0) {
                      args.HasError = true;
                      var messageArgs2 = { ExcludedDates: _.reduce(diff, function (memo, item) {
                              return memo + item.format("MM/DD/YYYY");
                          }) };
                      args.ErrorMessage = Utils.StringFce.format("Excluded days are not in range. '{ExcludedDates}'.", messageArgs2);
                      args.TranslateArgs = { TranslateId: 'ExcludedDaysMsg', MessageArgs: messageArgs2 };
                      return;
                  }
              }
docs: create validator function