| Event | Applies to | Start | End |
|---|---|---|---|
The three statutes and the rules applied
Assessment (ASED). IRC 6501(a) gives the IRS 3 years after the return was filed to assess, whether the return was filed early or late. A return filed before its due date is treated as filed on the due date (IRC 6501(b)(1)), and Forms 941, 943, 944 and 945 for a calendar year filed before April 15 of the next year are treated as filed on that April 15 (IRC 6501(b)(2)). The period is 6 years when gross income omitted is more than 25% of the gross income stated (IRC 6501(e)). There is no limit for a false or fraudulent return filed with intent to evade tax, or when no return was filed (IRC 6501(c)(1) and (3)). A Form 872 signed before the ASED extends it by agreement (IRC 6501(c)(4)); see Form 872 statute extensions and the assessment statute guide. A return filed before an extended due date is not an early return (IRM 25.6.1.9.3).
Collection (CSED). Once tax is assessed, IRC 6502(a)(1) allows collection by levy or court proceeding within 10 years after the assessment. Each assessment (the original return, a later deficiency, a penalty) carries its own date, so one tax year can have several CSEDs. A Form 900 waiver signed with a partial payment installment agreement extends it (IRC 6502(a)(2)). The CSED guide covers the transaction codes that change it.
Refund claims (RSED). IRC 6511(a) requires a claim within 3 years from the time the return was filed or 2 years from the time the tax was paid, whichever is later. The amount is capped by a look-back: tax paid within 3 years plus any extension period before the claim if it meets the 3-year rule, otherwise tax paid within 2 years (IRC 6511(b)(2)). Withholding and estimated tax are treated as paid on the due date without extensions (IRC 6513(b)). When the last day falls on a Saturday, Sunday or legal holiday, IRC 7503 moves it to the next business day; the calculator applies the District of Columbia legal holidays listed in IRM 25.6.1.6.18, including April 16 Emancipation Day. See refund claims for paid penalties and refund suits and claims.
Suspensions: you enter them
Many events stop the clock, and each statute defines its own start, end and tail. The calculator does not infer them; you enter the start and end dates for each event, including any tail the statute adds, and the calculator adds the days. Where periods overlap it counts the overlap once, following IRM 5.1.19.3: “Overlapping suspensions run concurrently; they are not cumulative.” A period that starts after the statute has already expired is ignored, because there is nothing left to suspend. Two that catch people: the CSED is not suspended while an installment agreement is in effect, only while a request is pending and for 30 days after a rejection or termination (IRM 5.1.19.3.5), and “The collection statute is not extended for equivalency hearings” (IRM 5.1.19.3.3), only for timely CDP hearings.
Checked against IRM examples
IRM 5.1.19.3 works an example: an assessment on 06/01/2009 (CSED 06/01/2019), a combat-zone suspension from 05/10/2014 through 08/28/2015 (including the 180-day tail) and an offer in compromise from 04/20/2015 through 11/16/2015 (including the 30-day rejection period). Entered as two overlapping CSED rows, the calculator merges them into one 555-day suspension and returns 12/07/2020, the IRM's answer. For refunds, IRM 25.6.1.10.3.3.1.4 says a 2024 return deemed filed April 15, 2025 can be claimed through Tuesday, April 18, 2028, because April 15, 2028 is a Saturday and Monday, April 17 is the observed Emancipation Day; the calculator returns April 18, 2028. Its second example, a 2022 return filed early on April 14, 2023 and treated as filed April 15, 2023, gives an RSED of Wednesday, April 15, 2026, so a claim on April 16, 2026 shows as late, as the IRM says.
Questions practitioners ask
Does a late-filed return still get only 3 years for assessment?
Yes. The 3 years under IRC 6501(a) run from when the return was actually filed, so a late return starts its own clock on the filing date. Only an early return is pushed to the due date.
Why does my transcript show a different CSED than the calculator?
The IRS computes the CSED from each assessment date and posts suspensions with transaction codes such as TC 520 (bankruptcy, CDP), TC 480 (offer) and TC 971 AC 043 (pending installment agreement). If a suspension is missing from your entries, or the IRS has not yet posted one, the dates will differ. The IRS's date is the one its systems use.
Is the 2-year refund period measured from the due date for withholding?
Withholding and estimated tax are treated as paid on the due date of the return without extensions (IRC 6513(b)), so for those payments the 2-year rule runs from that due date. Later payments run from the date actually paid.
Do disaster postponements change these dates?
They can. IRC 7508A periods are disregarded for many acts, and IRM 25.6.1.10.3.3 notes that for claims filed after December 26, 2025, disaster postponements are treated as extensions of time for filing in the refund look-back. Enter the postponed dates yourself; the calculator does not apply disaster relief automatically.