code: function (field) {
              var fields = this.fields;
              switch (field) {
              case WEEK_OF_MONTH:
                  fields[DAY_OF_MONTH] = undefined;
                  break;
              case DAY_OF_YEAR:
                  fields[MONTH] = undefined;
                  break;
              case DAY_OF_WEEK:
                  fields[DAY_OF_MONTH] = undefined;
                  break;
              case WEEK_OF_YEAR:
                  fields[DAY_OF_YEAR] = undefined;
                  fields[MONTH] = undefined;
                  break;
              }
          }
docs: roll the year of the given calendar field.
  @method rollYear
  @param {Number} amount the signed amount to add to field.
   
  roll the month of the given calendar field.
  @param {Number} amount the signed amount to add to field.
  @method rollMonth
   
  roll the day of month of the given calendar field.
  @method rollDayOfMonth
  @param {Number} amount the signed amount to add to field.
   
  roll the hour of day of the given calendar field.
  @method rollHourOfDay
  @param {Number} amount the signed amount to add to field.
   
  roll the minute of the given calendar field.
  @method rollMinute
  @param {Number} amount the signed amount to add to field.
   
  roll the second of the given calendar field.
  @method rollSecond
  @param {Number} amount the signed amount to add to field.
   
  roll the millisecond of the given calendar field.
  @method rollMilliSecond
  @param {Number} amount the signed amount to add to field.
   
  roll the week of year of the given calendar field.
  @method rollWeekOfYear
  @param {Number} amount the signed amount to add to field.
   
  roll the week of month of the given calendar field.
  @method rollWeekOfMonth
  @param {Number} amount the signed amount to add to field.
   
  roll the day of year of the given calendar field.
  @method rollDayOfYear
  @param {Number} amount the signed amount to add to field.
   
  roll the day of week of the given calendar field.
  @method rollDayOfWeek
  @param {Number} amount the signed amount to add to field.
   
  remove other priority fields when call getFixedDate
  precondition: other fields are all set or computed
  @protected