code: function(date, offset, period) {
          this._validate(date, this.minMonth, this.minDay,
              _exports.local.invalidDate || _exports.regionalOptions[''].invalidDate);
          return this._correctAdd(date, this._add(date, offset, period), offset, period);
      }
docs: Add period(s) to a date.
  Cater for no year zero.
  @memberof BaseCalendar
  @param date {CDate} The starting date.
  @param offset {number} The number of periods to adjust by.
  @param period {string} One of 'y' for year, 'm' for month, 'w' for week, 'd' for day.
  @return {CDate} The updated date.
  @throws Error if a different calendar used.