Math-Error Abatement Clock

By Forrest Baumhover, CFP®, EA · Last verified September 28, 2026

Enter the date printed on a CP10, CP11, CP11A, CP12, CP13, or CP16 notice and get the exact deadline to request abatement (asking the IRS, in writing, to undo the change) under IRC §6213(b)(2)(A) — 60 days from the notice date, adjusted under IRC §7503 when the 60th day falls on a weekend or legal holiday. This calculator only applies to those six notices — see “Which notices this covers” below before relying on it for any other notice.

Background on which notices this right applies to, and what changes on math-error notices starting November 25, 2026, under the IRS Math and Taxpayer Help Act: see The IRS Math and Taxpayer Help Act.

Which notices this covers

Six notices rest on IRC §6213(b)(1), the true math-or-clerical-error authority, and carry the full 60-day abatement right this calculator computes: CP10, CP11, CP11A, CP12, CP13, and CP16.

CP23, CP24, and CP25 rest on the narrower IRC §6201(a)(3) authority, which borrows the math-error assessment mechanism but explicitly excludes the §6213(b)(2) abatement right — there is no 60-day undo button on those three. CP101, CP102, CP111, and CP112 correct employment taxes, which sit outside deficiency procedures altogether, and CP268 is a credit-holding notice rather than a tax assessment — none of the five run through §6213(b)(1) either. The full legal analysis for every one of these, including the two Form 1042 notices, CP107 and CP117, is in the IRS Math and Taxpayer Help Act explainer.

How the deadline is computed

The calculator adds 60 calendar days to the notice date (IRC §6213(b)(2)(A)), then checks whether that 60th day is a Saturday, Sunday, or legal holiday. If it is, IRC §7503 moves the deadline to the next day that is none of those — which can cascade through more than one day in a row, the way a holiday observed on a Friday followed by a weekend does.

“Legal holiday” here means the District of Columbia’s legal holidays under D.C. Code §28-2701, which 26 CFR §301.7503-1 makes the standard for every IRC §7503 deadline nationwide — not just the taxpayer’s own state or local holidays. That list includes New Year’s Day, the Birthday of Dr. Martin Luther King Jr., Washington’s Birthday, District of Columbia Emancipation Day (April 16), Memorial Day, Juneteenth, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving, and Christmas — each already shifted to its own D.C.-law observed date when the calendar date itself falls on a weekend.

Two things the calculator does not model: Presidential Inauguration Day (a legal holiday only in the D.C. area, on a four-year cycle, next affecting January 20, 2029) and a one-off holiday a President proclaims after the fact. Neither is common enough, or knowable far enough in advance, to build into a date calculator — if a computed deadline falls close to either, confirm it before relying on it.

When to call a practitioner (a tax professional, such as a CPA or enrolled agent)

The computed deadline has passed, or falls within the next few days — confirm the notice date and the computation before relying on it, and act immediately if time remains.
You were serving in a combat zone or contingency operation, or your area had a federally declared disaster, around the time of the notice — either can extend this deadline beyond what the calculator shows.
The notice is CP23, CP24, CP25, CP101, CP102, CP107, CP111, CP112, CP117, or CP268 — this calculator’s 60-day right does not apply the same way, if at all; see the notices link above.
You are not sure whether the underlying correction is actually right — pull the account transcript and compare it against the return before deciding whether to dispute or to pay.

Common Questions

Where do I find the notice date to enter?

Use the date printed on the notice itself — most math-error notices (CP10, CP11, CP11A, CP12, CP13, CP16) print it in the upper right of page one. The 60 days run from that date, not from the day the notice arrived or the day you open it (IRC §6213(b)(2)(A)).

Why did the calculator move my deadline later than 60 days?

IRC §7503 pushes a deadline that falls on a Saturday, Sunday, or legal holiday to the next day that is none of those. The calculator applies that rule using the District of Columbia’s legal holidays (26 CFR §301.7503-1 incorporates D.C. Code §28-2701), including the shift D.C. law itself applies when one of those holidays falls on a weekend.

Does mailing my request by the deadline count, or does the IRS have to receive it by then?

Mailing counts. Under the IRC §7502 mailbox rule, a request postmarked by the U.S. Postal Service — or marked by a designated private delivery service such as FedEx or UPS on one of their qualifying services — by the deadline is timely even if the IRS receives it afterward. Keep proof of the postmark or the delivery service’s tracking record.

Does every math-error notice carry this 60-day right?

No. CP10, CP11, CP11A, CP12, CP13, and CP16 all rest on IRC §6213(b)(1) and carry the full right this calculator computes. CP23, CP24, and CP25 rest on the narrower IRC §6201(a)(3) authority, which borrows the assessment mechanism but not the §6213(b)(2) abatement right. CP101, CP102, CP111, and CP112 (employment tax) and CP268 (a credit-holding notice) do not run through §6213(b)(1) at all. See the IRS Math and Taxpayer Help Act explainer, linked below, for the full breakdown.

What if I am serving in a combat zone or my area has a federally declared disaster?

Either can extend this deadline well past what the calculator shows — IRC §7508 for combat zone or contingency-operation service, IRC §7508A for a federally declared disaster — but how much depends on facts (deployment dates, or the specific disaster declaration and the postponement period IRS guidance sets for it) this tool cannot look up. Check IRS.gov’s disaster relief announcements or talk to a practitioner before treating the calculator’s date as final in either situation.

What happens if I miss the deadline?

The assessment stands, and the formal right to force the IRS to abate it and start over with a real deficiency notice (the one that carries Tax Court rights) is gone through this channel. Several of the notices’ own IRS.gov pages note the IRS will still informally consider a late request with supporting documentation, but without the same guarantee — see each notice’s own page (linked below) for what it says about a late response.

More from the Desk

More practitioner writing from Forrest Baumhover, CFP®, EA at The Federal Tax Desk on Substack.

Sources

Read The Federal Tax Desk: every Deep Dive and Cheat Sheet, with primary-source citations →