Specialist US UK Tax Services: The Tax Year Mismatch

The UK tax year ends 5 April; the US tax year ends 31 December. For a dual filer catching up several years at once, that offset governs everything, which is why Specialist US UK tax services begin by apportioning PAYE income into calendar years and matching every pound of UK tax to the US year it actually belongs in.

Most guides on filing late in both countries describe the engagement: which disclosure route to use, how many years, what the penalties look like. This one describes the machinery underneath it. When a client comes to Jungle Tax with four or five open years on both sides, the disclosure route is rarely the hard part. The hard part is that the two revenue authorities measure time differently, and unless the numbers are rebuilt on a common timeline before anything is filed, the UK tax paid will not land where the IRS can credit it. The return will still be accepted. It will simply produce a US liability that should never have existed, and a pool of unusable foreign tax credits that quietly ages out.

Why do the US and UK tax years not line up?

The UK personal tax year runs from 6 April to the following 5 April, a survival of the pre-1752 calendar and subsequent adjustments that HMRC has never seen reason to change. The United States taxes individuals on the calendar year, 1 January to 31 December. The result is a permanent 96-day overhang: every UK tax year straddles two US tax years, and every US tax year draws from two UK tax years.

For a US-resident American with no UK exposure this is trivia. For a US citizen or green card holder living and earning in the UK, it is the single structural feature that determines whether double taxation relief works. The US-UK treaty and the domestic foreign tax credit rules both operate on a principle of matching: relief is given for foreign tax imposed on the same income, in the same period. Two misaligned periods make that matching an act of reconstruction rather than transcription.

Feature	United States (IRS)	United Kingdom (HMRC)
Tax year	1 January to 31 December	6 April to 5 April
Primary individual return	Form 1040, plus 1116, 8938, FinCEN 114	Self Assessment SA100, plus SA106 foreign pages
Standard filing deadline	15 April; automatic extension to 15 June for those abroad; October on request	31 January following the end of the tax year (online)
How employment tax is collected	Withholding and quarterly estimates, balance settled at filing	PAYE in real time, with any balance settled through Self Assessment
Interim payments	Quarterly estimated tax	Payments on account due 31 January and 31 July
Core evidence document	W-2, 1099, brokerage statements	P60, P45, P11D, payslips, SA302
Relief mechanism	Foreign tax credit on Form 1116, or the foreign earned income exclusion	Foreign Tax Credit Relief under HS263 and the treaty
Functional currency	US dollars	Pounds sterling

What actually breaks when you catch up several years at once?

A single late year can often be forced into shape by approximation. A five-year catch-up cannot, because the errors compound and reconcile against each other. Three specific mechanical failures account for most of the damage we see in files that arrive from generalist preparers.

The P60 is the wrong shape for a US return

A P60 reports cumulative pay and tax for a UK tax year. It tells you what an employee earned between 6 April and 5 April, and what PAYE was deducted across that span. It does not tell you what was earned in a calendar year, and it cannot be used as if it did. Yet the most common error in DIY and low-cost expatriate filings is exactly that: the 2024/25 P60 is dropped whole into the 2024 Form 1040 because the years share a number.

The effect is not neutral. It shifts roughly a quarter of one year's income and tax into the wrong US period, and it does so in the same direction every year, so the distortion never washes out. In a rising-salary or bonus-heavy year the error is material. Where a bonus or vesting event falls in, say, February, it sits in one UK tax year and a different US one, and reporting it by P60 places it in a US year in which the taxpayer may have had no matching credit capacity at all.

PAYE deducted is not UK tax finally due

The second failure is subtler. Even correctly apportioned, PAYE deducted in a period is a provisional figure. It is adjusted by coding notices, by underpayments carried forward from earlier years, by pension relief given at source or claimed through Self Assessment, and by the eventual Self Assessment computation. For a higher or additional rate taxpayer with dividends, carried interest, rental income or partnership profits, the definitive UK liability for a tax year is only fixed when the SA302 is produced, often long after the P60.

A US return prepared from PAYE figures alone therefore claims a credit for a number that later changes. If the change is upward, credit is left on the table; if downward, the return is overstated and the foreign tax redetermination rules require the position to be corrected. Neither outcome is acceptable in a disclosure that is supposed to demonstrate reasonable care.

Self Assessment payments bunch into the wrong US year

The third failure is created by the UK payment calendar itself. Payments on account fall on 31 January and 31 July, and a balancing payment falls on the following 31 January. A taxpayer settling several open UK years at once during a catch-up may make three or four years of UK payments inside a single calendar month. On a cash basis, every one of those payments is a foreign tax paid in one US year.

The consequence is stark: one US year shows an enormous credit with nothing like enough US tax to absorb it, and the surrounding years, which carried the income, show none. Credits stranded this way are not lost immediately, but they enter a carryback and carryforward queue that many taxpayers never work through.

How do you apportion a UK tax year into US calendar years?

The correct method is to rebuild from payslips, not to prorate the P60 by nine-twelfths and three-twelfths. Straight-line apportionment is a fallback for genuinely level salary with no variable pay; it is indefensible where bonuses, share vesting, salary changes, redundancy payments or termination sums are present, because those are point-in-time events that belong in a specific month.

The working method we use across a multi-year catch-up is as follows.

• Collect monthly payslips for the whole exposure window, not just the years being filed. If the earliest US year being filed is 2022, you need UK payroll data from January 2022, which means the 2021/22 P60 and its payslips as well.
• Tag every line to the month of receipt: basic pay, bonus, commission, employer pension contributions (which matter for the UK but are treated very differently for US purposes), benefits in kind from the P11D, and PAYE and National Insurance deducted.
• Rebuild each calendar year as January to December from those monthly lines. The result should reconcile: the sum of your calendar-year rebuilds across a span must equal the sum of the P60s across the same span, with only the opening and closing stubs differing.
• Rebuild UK tax on the same basis, then reconcile it forward to the SA302 for each UK year so that the definitive liability, not the provisional PAYE, drives the credit.
• Reconcile the residual. Any difference between apportioned PAYE and final UK liability is allocated to the income that produced it, in the period that income arose, which is the same matching principle HMRC applies in its own foreign tax credit relief guidance.

A short worked illustration makes the shape of it clear. Assume a client earning a level 180,000 pounds with a 120,000 pound bonus paid each February.

Period	Source document	Employment income	Belongs in US year
Apr 2024 to Dec 2024 (nine months salary)	2024/25 P60	135,000 pounds	2024
Jan 2025 to Mar 2025 (three months salary)	2024/25 P60	45,000 pounds	2025
February 2025 bonus	2024/25 P60	120,000 pounds	2025
2024/25 P60 total	Single UK tax year	300,000 pounds	Split across two US years

Filing the P60 total of 300,000 pounds into US year 2024 would overstate 2024 income by 165,000 pounds and understate 2025 by the same amount. The credit position moves with it, and because the bonus is taxed at the UK additional rate while the salary stub is not, the tax follows the income unevenly. This is precisely the sort of error a generalist preparer does not see, because nothing on the face of the return looks wrong.

Which exchange rate should you use, and does consistency matter more than accuracy?

Both matter, but consistency is what survives examination. The IRS position is that you translate items into US dollars using the rate prevailing when you receive, pay or accrue the item, and where several rates exist you use the one that most properly reflects the income. Its guidance on foreign currency and currency exchange rates also publishes yearly average rates, which are accepted for income received evenly through the year.

In practice a defensible multi-year catch-up uses a documented hierarchy: spot rate on the date of receipt for discrete events such as bonuses, vesting, disposals, dividends and the actual date each UK tax payment cleared, and the published yearly average for regular monthly salary. What is not defensible is switching method between years, or using the yearly average for income and a favourable spot rate for the tax on it. That asymmetry is visible on the face of Form 1116 and is one of the first things a reviewer tests.

A related trap: sterling weakness or strength between the year the income arose and the year the UK tax was paid can create or destroy credit on its own. Two identical years in sterling can produce different dollar credits purely on currency movement. In a catch-up spanning several years of volatile rates, this needs to be modelled, not discovered.

Paid or accrued? The election that decides which US year the UK tax lands in

This is the fulcrum of the whole problem. A cash-basis US taxpayer claims foreign tax credits in the year the foreign tax is paid. Because UK tax on a given slice of income is frequently paid in a later calendar year, sometimes two later once payments on account and balancing payments are counted, the cash basis systematically separates the income from its credit.

The alternative is to elect to claim credits on the accrued basis, so that UK tax is credited in the US year in which the income arose and the liability accrued, regardless of when it was actually paid. Elected properly, this largely dissolves the mismatch: apportioned UK income and apportioned UK tax sit in the same US year, and Form 1116 does what it was designed to do. The IRS sets out the mechanics on its page about Form 1116.

Three cautions, all of which matter disproportionately in a catch-up.

• It is a lasting commitment. Once made, the accrual election binds subsequent years and cannot be revoked at will. It should never be made for the convenience of a single year.
• Timing of the election is restrictive. The rules constrain when a cash-basis taxpayer can first adopt the accrued basis, and the interaction with amended and delinquent returns is technical. Where a taxpayer has already filed some years on a paid basis and is now filing others late, the election needs to be positioned deliberately at the start of the engagement, not retrofitted at the end.
• Accrual demands better records. You must be able to compute the accrued UK liability for each US calendar year, which returns you to the apportionment work above. There is no shortcut in which accrual saves you from rebuilding the numbers.

Where the accrual basis is unavailable or unwise, the mismatch has to be managed rather than removed, through carryback and carryforward and occasionally through treaty re-sourcing where the income can properly be treated as foreign source for credit purposes.

How should a multi-year catch-up be sequenced?

Sequencing is where the mismatch stops being an accounting problem and becomes a project management one. The governing rule is simple: the UK numbers must be final before the US returns are filed, because the US return depends on UK tax as an input and the UK return does not, in the ordinary employment case, depend on US tax at all.

The order we work in is:

• Establish the UK residence and domicile position for every open year. The Statutory Residence Test result for each year determines what HMRC can tax, and therefore what credit exists at all. Following the 2025 reform of the taxation of foreign income and gains, older remittance-based positions do not apply in the same way to newer years, so a catch-up spanning the transition may sit under two different UK regimes.
• Prepare and quantify the UK Self Assessment returns for all open UK years, even where they will ultimately be filed simultaneously. Quantification precedes submission.
• Build the bridge schedule that converts UK-year income and UK-year tax into US calendar years, in dollars, with the rate source documented line by line.
• Model the US years both ways, paid basis and accrued basis, across the whole window, including carryback and carryforward, before choosing.
• File the UK returns, then the US package, with the information returns (FBAR, Form 8938 and any 5471, 8621 or 3520 exposure) prepared from the same reconstructed data rather than assembled separately.

Mapping the streamlined window onto UK tax years

The IRS streamlined filing compliance procedures require, for taxpayers resident outside the United States, three years of delinquent or amended returns and six years of FBARs, accompanied by a certification of non-wilful conduct. Note what that means on a UK timeline: three US calendar years touch four UK tax years, and six FBAR years touch seven. The data-gathering window is always wider than the filing window, and a catch-up scoped to three years of records will be short at both ends. Our team covers the disclosure route itself in detail through our IRS streamlined filing practice.

Stranded credits: carryback, carryforward and re-sourcing

When the mismatch has already done its damage, excess credits do not simply vanish. Foreign tax credits carry back one year and forward ten, absorbed oldest first against available limitation. In a catch-up this creates a specific and often overlooked opportunity, and a specific trap.

The opportunity: a credit arising in the earliest year of a streamlined package may carry back to a year outside the package, a year already filed. That can support an amended return and a refund claim, if the claim period remains open. The extended claim period that applies to foreign-tax-related adjustments is materially longer than the ordinary refund window, which is why credit-driven amendments are sometimes still available long after taxpayers assume the door has shut.

The trap: if the mismatch is left uncorrected, fresh excess credits are generated every single year. Each year's current credits consume the whole limitation first, and the oldest layer of the pool ages toward expiry untouched. A client can hold a very large credit balance and still pay US tax annually. The balance is not an asset if the structure guarantees it will never be used.

Where else does the mismatch bite?

Employment income is the clearest case but not the only one. In a high-net-worth file, the mismatch is usually worst outside payroll.

• UK pensions. Contributions, growth and lump sums are treated under treaty provisions that operate on their own timing logic, and annual allowance charges arise on UK-year events that must still be placed in a US calendar year.
• ISAs. Tax free to HMRC, fully taxable to the IRS, and frequently holding non-US funds that raise passive foreign investment company reporting. Because there is no UK tax at all, there is no credit to strand, but the income still has to be apportioned to the correct US year, and a 5 April statement will not do that for you.
• Capital gains. A disposal on 20 March sits in one UK tax year and one US year, but the UK capital gains tax on it is payable at a date that may fall in a different US year again. UK residential property disposals carry their own accelerated reporting and payment window, which pulls the payment date forward and changes the cash-basis credit year.
• Dividends and interest. Payment dates rarely align conveniently with either year end, and UK dividend taxation applies rates and allowances by UK year that have to be unpicked to attribute tax to a calendar-year slice.
• Rental income. The UK property business computation is a UK-year computation; the US Schedule E is a calendar-year computation with different depreciation, a different treatment of finance costs and different allowable expenses.

Do the information returns follow the same timeline?

No, and this catches people out. FBAR and Form 8938 test account balances by reference to the US calendar year: the maximum value during the calendar year, and in the case of Form 8938 also the position at 31 December. A UK bank or platform statement produced to 5 April gives you neither figure. Reporting from UK-year statements is a common source of understated maximum balances, which is exactly the kind of inaccuracy that undermines a non-wilfulness position. If you want to understand the exposure arithmetic before you engage, our FBAR penalty calculator models it.

What does HMRC expect on the UK side?

Symmetry matters. Where a UK resident has US-source income taxed in the United States, whether US rental property, US partnership income, certain US pensions or, for accidental Americans, US tax arising purely from citizenship, HMRC gives Foreign Tax Credit Relief on the same matching principle. Its Self Assessment helpsheet on relief for foreign tax paid (HS263) sets out the computation, including the rule that relief cannot exceed the UK tax attributable to that same income and that the credit follows the income to which it relates.

A treaty point routinely missed on the UK side: HMRC does not generally allow relief for US tax that arises only because of US citizenship, because the treaty's saving clause and its associated relief provisions govern that outcome instead. Getting this wrong in a UK catch-up produces an overclaim in HMRC's system while the corresponding US return underclaims, which is the worst of both. Our UK tax services and dual-handler team reconcile both sides within one computation rather than two.

The bridge schedule: the one working paper that holds it together

Every properly run US-UK catch-up produces a single reconciliation schedule from which both returns are built. At minimum it should carry, for each month across the whole exposure window: the sterling amount by income category; the source document; the UK tax year it falls in; the US tax year it falls in; the exchange rate applied and its published source; the dollar equivalent; the UK tax attributable; and whether that tax is credited on a paid or an accrued basis.

Two returns, two currencies and two calendars reconcile to one grid. When HMRC or the IRS asks a question three years later, and in disclosure cases they sometimes do, that grid is the answer. Without it the file cannot be defended, only re-created.

Common mistakes we correct in inherited files

• Dropping a P60 whole into the US year that shares its number.
• Claiming credit for PAYE deducted rather than the final UK liability per the SA302.
• Switching exchange rate methodology between years, or between income and tax within a year.
• Making or omitting the accrual election without modelling the whole window first.
• Scoping the records gather to the filing window rather than the wider data window apportionment requires.
• Preparing FBAR and Form 8938 from 5 April statements.
• Filing the US returns before the UK liabilities are final, then never revisiting them.
• Treating a large foreign tax credit carryforward as a stored asset rather than as a symptom.

None of these is exotic. All of them are structural, and all of them are the direct consequence of two tax authorities that measure the same year differently. You can read more on adjacent cross-border issues in our guides library.

Speak to us in confidence

If you are facing several open years in both countries and want them reconstructed properly, on one timeline, one currency grid and one credit strategy, filed in the right order, contact our cross-border team for a confidential, no-obligation consultation. We will tell you what the exposure genuinely is, how the work should be sequenced, and what a defensible file looks like, before anything is submitted to HMRC or the IRS.