In the case of an association which converts from a mutual association to a stock association, for a period of up to five years from the date of conversion, special meetings of the stockholders of a stock association may be called at any time by the president, or the board of directors, and shall be called by the president, vice president, or secretary upon the written request of stockholders holding of record in the aggregate at least 20 percent of the outstanding shares of the association, which request shall state the purpose of the meeting and shall be delivered at the principal office of the association addressed to the president. The date for any

special meeting called by stockholders shall be set by the board of directors and shall be not less than 60 nor more than 90 days after the date of receipt of the request.