A filed tax return does not stay open indefinitely. Statutory periods limit how long the authority has to assess additional tax and how long a taxpayer has to claim a refund.
The general assessment period
The standard period for assessing additional tax runs from the date the return was filed, generally three years, after which the year is normally closed.
A return filed before the due date is treated as filed on the due date, so filing early does not start the clock earlier than the statutory deadline.
Closing means the authority generally cannot assess more tax for that year, which is why record retention advice is commonly tied to this period.
When the period is longer
Substantial understatement of gross income extends the assessment period, doubling it in the circumstances the statute defines.
Certain international information reporting failures can extend the period for the entire return rather than only for the item reported incorrectly.
Where no return was filed or where fraud is involved, no limitation period runs at all, leaving the year open indefinitely.
The separate refund clock
Claiming a refund has its own deadline, generally the later of a period from filing or a period from when the tax was paid.
Amounts withheld from wages are treated as paid on the return due date, which affects how the deadline is computed for a year with substantial withholding.
A refund claim filed after the deadline is generally barred regardless of whether the overpayment was genuine, which makes the timing consequential.
What suspends the clock
Agreements to extend the period exist, and are sometimes requested during an examination to avoid a protective assessment before the review is complete.
Bankruptcy proceedings, certain appeals and periods when a taxpayer is outside the country can suspend the running of the period under specified conditions.
Because suspension depends on facts, the actual expiration date of a particular year is often not simply three years from filing.
Why amended returns complicate it
Filing an amended return does not generally restart the assessment period, though a special rule permits assessment of amounts shown on a late-filed amendment.
State periods run separately and are not always the same length, so a year closed federally may remain open at the state level.
These interactions are why the limitation question is answered by a practitioner examining a specific file rather than by a general rule. Periods and their exceptions change over time and vary by jurisdiction.