A foundation must keep books, reports, and file returns based on an annual accounting period called a tax year. A tax year is usually 12 consecutive months. There are two kinds of tax years: Calendar tax year This is a period of 12 consecutive months beginning January 1 and ending December 31; or Fiscal tax year This is a period of 12 consecutive months ending on the last day of any month except December. Return to Life Cycle of a Private Foundation