The Bank of Canada daily noon rates are used to derive all periods.
The "period" input simply manipulates the days included. "Average for day" will provide the day's noon rate. "Average for month" will provide the average of ~30 noon rates. This is the same method used by the Bank of Canada to provide monthly and annual rates.
The earliest data is from January 2, 1970 and the latest data is specified within the worksheet. All periods are available for which official data exist.