Tax Year You must file your tax return on the basis of an annual accountingperiod called a tax year. If you have not previously established afiscal tax year, your tax year is the calendar year. A calendar yearis 12 consecutive months ending on December 31. If you have previouslyestablished a regular fiscal year (12 consecutive months ending on thelast day of a month other than December, or a 52-53 week year)and are considered to be a U.S. resident for any calendar year, youwill be treated as a U.S. resident for any part of your fiscal yearthat falls within that calendar year.