A notice of deficiency formally notifies the taxpayer of the IRS's determination that additional tax is owed and starts a strict statutory clock: the taxpayer generally has 90 days from the date of the notice (150 days if the notice is addressed outside the United States) to file a petition in the Tax Court. The IRS is generally barred from assessing the proposed deficiency during that window and during the pendency of any timely-filed Tax Court proceeding, which is what preserves the taxpayer's ability to litigate before paying.

The notice carries a presumption of correctness, meaning the taxpayer generally bears the initial burden of showing the IRS's determination is wrong, subject to the limited burden-shifting rule discussed in the burden-of-proof framework. Missing the filing deadline is largely fatal to Tax Court jurisdiction over that deficiency: courts have treated the deadline as strictly enforced, leaving a taxpayer who misses it to pay the assessment and pursue a refund suit instead.

Because the deficiency-notice deadline is fixed by statute and does not bend for settlement negotiations, litigation strategy, or ordinary excusable delay, it functions in a case model as a hard, non-negotiable deadline rather than a soft procedural target. Juricratic represents it accordingly: unlike settlement-leverage dials that respond to negotiation dynamics, the deficiency-notice deadline is modeled as an unmodifiable structural constraint that a simulated strategy cannot talk its way around.

A taxpayer or representative who receives a notice of deficiency calendars the 90-day (or 150-day) deadline immediately as the controlling date for the entire matter, since it determines whether the Tax Court's prepayment forum remains available at all, independent of how settlement discussions with IRS Appeals are proceeding.