_modifyTimestampBy(amount, unit, preserveDate) {
      // Modify the current timestamp
      this._safelyModifyTimestampBy(amount, unit, preserveDate);
      this._updateSecondTimestamp();
      // Notify the calendar to update its timestamp
      this.fireEvent("navigate", {
        timestamp: this.timestamp
      });
    }
Modifies the timestamp by a certain amount of days/months/years.
@param { number } amount
@param { string } unit
@param { boolean } preserveDate whether to preserve the day of the month (f.e. 15th of March + 1 month = 15th of April)
@private