A tax year is an annual accounting period that an CDA tax payer uses to determine their tax liability. A calendar year is January 1st through December 31st. Individuals, including an Idaho sole proprietorship, always uses the calendar year.

A fiscal year is a tax year that starts and ends in another month, except December. You can opt to use a fiscal year if you can prove to the IRS that your natural Idaho business cycle conflicts with the calendar year.