The year of the income and the year a return is filed are not interchangeable.
A tax year is the annual accounting period used to report income and expenses. A calendar-year 2025 return is commonly filed during 2026. That filing date does not turn the return into a 2026 tax return.
Match brackets, deductions, forms and effective dates to the period under study. A current IRS webpage can display a newer revision, and fiscal or short years require extra care. California conformity must be checked separately rather than assumed from the federal result.
Work through an example
Illustration: comparing a 2025 return filed in 2026 with a projection for 2026 requires two sets of year-specific assumptions. Label both scenarios before comparing tax amounts.
Return to the sources
Tax years and annual accounting periods
Distinguish calendar, fiscal and short tax years before selecting a form or applying a threshold.
Prior-year forms and instructions
Use the forms and instructions for the year being researched, rather than relying on the current page alone.
Partnership tax years and timing
Follow the rules that connect the partnership’s year, the partner’s year and changes in ownership.
California conformity to federal law
A federal tax change does not automatically determine the California result. Review the state’s specific treatment.
First edition: November 15, 2026. Educational scope and current links reviewed for this release. Source text and effective dates control.