The business year of a foreign bank branch office is from April 1 to March 31 of the following year, or is the same period as that of the business year of the foreign bank with which the foreign bank branch office is affiliated (limited to a business year with a period of one year, which commences on the first day of a month); provided, however, that in changing the first day of the business year, the last business year before the change runs up until the day before the first day of the first business year after the change.