The assessment of taxes herein made and the returns required therefor shall be for the year ending on December 31. If the taxpayer, in exercising a privilege taxable under this chapter, keeps the taxpayer’s books reflecting the same on a basis other than the calendar year, the taxpayer may, with the assent of the department of taxation, and upon the direction of the department shall, make the taxpayer’s annual returns and pay taxes for the year covering the taxpayer’s accounting period as shown by the method of keeping the taxpayer’s books.