_modifyTimestampBy(amount) {
      // Modify the current timestamp
      this._safelyModifyTimestampBy(amount, "year");
      // Notify the calendar to update its timestamp
      this.fireEvent("navigate", {
        timestamp: this.timestamp
      });
    }
Modifies timestamp by a given amount of years and, if necessary, loads the prev/next page.
@param { number } amount
@private