_modifyTimestampBy(amount, preserveDate) {
      // Modify the current timestamp
      this._safelyModifyTimestampBy(amount, "month", preserveDate);
      // Notify the calendar to update its timestamp
      this.fireEvent("navigate", {
        timestamp: this.timestamp
      });
    }
Modifies timestamp by a given amount of months and,
if necessary, loads the prev/next page.
@param { number } amount
@param { boolean } preserveDate whether to preserve the day of the month (f.e. 15th of March + 1 month = 15th of April)
@private