The term fiscal year means any period not longer than 12 calendar months, or any shorter period beginning on the date the taxpayer becomes subject to tax and ending on the last day of any month other than December. (See section 17-1.3 of this Title - Fiscal year taxpayers.) The term fiscal year also includes the 52-53 week accounting period if such period has been elected by the taxpayer. (See section 17-1.4 of this Title - 52-53 week fiscal year taxpayers.)