Ninety days, printed on the letter, and the IRS cannot move it.

What the notice means

The letter arrives by certified mail and goes by several form numbers: CP3219A after an unanswered CP2000, Letter 531 or Letter 3219 after an audit, and others. Whatever the number, the first page states a deficiency for a specific tax year, lists penalties, and gives a date labeled as the last day to file a petition with the United States Tax Court.

The IRS sends this notice when an examination or an underreporter case has ended without agreement, or without any response. Until the notice is issued and the 90 days run, the IRS is barred by law from assessing the tax or collecting it.

The deadline

Ninety days from the notice date. The count includes weekends and holidays, and the final date is printed on the notice so you do not have to calculate it. A petition is on time if it is filed electronically by that date or mailed with a proper postmark by that date.

Your options during the 90 days

• Petition the Tax Court. This keeps the tax from being assessed while the case is pending, and it does not require payment first. After a petition is filed, most cases are referred to the IRS Independent Office of Appeals for settlement discussions, and most are resolved without a trial. Cases with $50,000 or less in dispute per year can use simplified small tax case procedures. For North Texas taxpayers, the Tax Court holds its trial sessions in Dallas.
• Agree and sign the waiver. If the IRS is right, signing Form 5564 stops some interest from accumulating and moves the account to billing, where payment arrangements can be made.
• Keep working with the IRS, with a petition on file. If you have documents the IRS has not considered, they can still be reviewed. The petition protects you if that review does not finish in time.
• Pay, then claim a refund. You can pay the full amount, file a refund claim, and sue in federal district court or the Court of Federal Claims if it is denied. This route requires the money up front and is rarely the first choice.

What happens if you ignore it

On day 91 the deficiency becomes final. The IRS assesses the tax, penalties, and interest, and sends a bill. From there the path runs through collection notices to a final notice of intent to levy. You generally cannot contest the amount at a later collection hearing, because you already had the opportunity and did not use it. The remaining routes to challenge the tax itself, such as audit reconsideration or a refund suit, are slower and narrower.

If you never received the notice

A notice of deficiency is valid if the IRS mailed it to your last known address, whether or not you actually received it. If the IRS used a wrong address despite having a newer one, the assessment that followed may be open to challenge. That question turns on mailing records and address history, and it is worth having reviewed if collection began on a tax you never had a chance to dispute.

When a lawyer matters

This is the notice where legal help matters most, because it is the only point where the dispute can move from an agency to a court, and because the decision has to be made inside a fixed window. A review should cover whether the IRS's numbers are right, what it would take to prove otherwise, whether penalties were properly asserted, and whether petitioning or agreeing costs less in the end. If the 90 days are nearly gone, the first job is protecting the deadline. Everything else can be sorted out afterward.