Six dates, two countries, and none of them line up

The British year runs from 6 April to 5 April. The American one is the calendar year, closing on 31 December. Every downstream date inherits that gap, which is why no two deadlines in the two systems ever coincide.

Four, and GOV.UK lists them on its Self Assessment deadlines page. The first falls on 5 October, when anyone newly within Self Assessment has to notify HMRC for the year just ended.

Paper returns follow on 31 October. Online returns and the payment itself both fall on 31 January.

Registering late carries its own twist. HMRC issues a fresh filing date roughly three months out, while leaving the January payment date exactly where it was.

Yes, automatically, and the IRS confirms the position on its when to file page. Citizens living outside the country receive an extension to 15 June without applying for anything, and a further extension to 15 October is available on request.

A further extension to 15 October is available on request.

Neither extension moves the payment date. Interest runs on unpaid tax from April regardless of how long you have to file.

Because it hides behind the filing date. People hear that they have until June or October and assume the whole obligation shifts with it, while interest accrues quietly from April on anything unpaid and compounds while they wait for information.

Interest accrues quietly from April on anything unpaid, and it compounds while you wait for information.

The British equivalent catches people the same way. An extended deadline to file after late registration does not push the 31 January payment date.

So the rule to carry is simple: estimate early, pay early, file when you have the figures.

They are the British instalment system, and they routinely shock first-time filers. Where your liability passes the threshold, HMRC asks for payments towards next year's bill in January and July, so a first Self Assessment year can produce a demand far larger than that year's tax alone.

So a first Self Assessment year can produce a demand far larger than the tax for that year alone.

Our article on payments on account explains how they are calculated and when they can be reduced.

No, it runs on its own schedule with FinCEN rather than the IRS. The annual deadline aligns with April and carries an automatic extension that needs no request, which means the two filings drift apart in people's diaries surprisingly easily.

That means it is possible to file the FBAR and forget the return, or the reverse.

Treat them as two separate diary entries rather than one. Our comparison of Form 8938 and the FBAR sets out what each one covers.

Then four more dates join the list, spread across the year. Estimated payments matter most to anyone whose American tax is not collected through withholding, which describes almost every self-employed person abroad, and underpayment charges apply even where the final return is correct.

Underpayment charges apply even where the final return is correct and filed on time.

Our article on estimated tax while living abroad covers the quarterly timetable and the safe harbours.

Filing dates move and payment dates do not. That one sentence covers most of what goes wrong here. Both systems allow extra time to submit a return, and neither system delays the date the money itself falls due.

Extensions exist in both systems for the return itself. Neither system extends the date the money is due.

Weekends and bank holidays shift a deadline to the next working day in practice, though relying on that is a poor habit.

Where a date falls awkwardly, treat the earlier one as real and the later one as a contingency.

Awkwardly, and permanently. A British tax year straddles two American ones, so any British figure has to be re-cut before it fits an American return. A P60 for the year to 5 April 2026 covers three months of one American year and nine months of the next.

A P60 for the year to 5 April 2026 covers three months of the 2025 American year and nine of 2026.

That is why we ask for payslips rather than annual summaries. The summary answers a British question and cannot answer an American one.

Our guide to the UK tax year explains the British calendar in more detail.

Britain applies a fixed penalty once the filing date passes, with further charges as time goes on, plus interest on unpaid tax. America charges separately for failing to file and for failing to pay, which is why filing on time matters even when you cannot pay.

The two regimes are independent, so being late in one has no effect on the other. GOV.UK's guidance on what to do if you cannot pay covers the British side.

Relief exists in both systems where there is a reasonable excuse or reasonable cause.

Our article on reasonable cause penalty relief covers the American route.

Not all of them, which is worth establishing before you diary anything. Someone on PAYE with no other income may have no British filing obligation at all, while still carrying a full American one every single year.

A British return arrives once you have untaxed income, a let property, or foreign income to declare here.

So the honest first question is which system actually wants a return from you. The tax deadlines that follow depend entirely on that answer.

Everything arrives at once, in an unfamiliar order. You may have a British return covering part of a year, an American return covering all of one, and a first FBAR for accounts opened on arrival.

October's notification date is the one most often missed that year, because nobody thinks to mention it.

Work backwards from January rather than forwards from your arrival. British obligations then surface while there is still time to act on them.

Then add its calendar and treat the other two as fixed. Americans who move on from Britain often keep a UK obligation for a let property while acquiring a new one somewhere else entirely.

Three sets of tax deadlines are manageable in one diary and unmanageable in three.

Put every date in a single calendar, marked by country, and review the whole thing each autumn.

Put five entries in your calendar and leave them there. Early September, to gather British documents. 5 October, as a registration check. Early January, to pay the British bill.

Then early April for an American estimate, and October as the final filing date.

Set each one a fortnight before the real date. Every deadline in this article is easier met with two weeks in hand than with two days.

In our practice, the clients who diary backwards from the payment dates almost never pay a penalty.

Self-employment adds dates at both ends. In Britain, Self Assessment applies from the first year of trading and payments on account usually follow in the second.

In America, quarterly estimated payments replace the withholding an employer would have handled, and self-employment tax rides alongside the income tax.

So a freelance year carries roughly ten dates rather than six. None of them is difficult individually.

The failure mode is always the same: no single calendar holds all of them, so one gets missed each year.

Do this once a year, in the autumn.

The dates themselves do not move, though whose information you need does. A joint American return covers both spouses, so both sets of documents have to be ready before it can go in.

Where one spouse is not American, the choice of filing status matters more than the calendar.

Plan for the slower half of the household. In practice the return goes in when the last document arrives, not when the first one does.

The dates your information actually arrives. A P60 lands in May or June, a P11D later still, and investment platform statements turn up whenever they turn up. So the working deadline for a British return is not 31 January at all, but the week your slowest document finally appears.

So the working deadline for a British return is not 31 January but the week your last document turns up.

Chase the slow ones in the autumn rather than in January. That single habit moves most people from filing late to filing early.

Our clients who request documents in September are rarely the ones asking about penalties in February.

Take an illustrative year. An American in Leeds has UK employment plus a small freelance income, so she files in both countries.

She registers with HMRC by 5 October, files online and pays by 31 January. Her American return uses the automatic June extension, though she pays her estimated American tax in April to stop interest running.

Her FBAR goes in separately. The dates are fixed; the figures are illustrative, and the pattern repeats annually once set up.

First, treating a filing extension as a payment extension. Second, missing the 5 October registration date entirely.

Third, assuming the FBAR rides along with the tax return. Fourth, using a P60 as though it covered an American year.

Fifth, waiting for perfect figures before paying anything. Sixth, forgetting payments on account in a first British filing year.

We run a single calendar covering both countries for each client, working backwards from the payment dates rather than the filing ones. That ordering is what keeps interest off the account.

Our treaty relief service covers the returns themselves, and we prepare both sides so the figures agree.

Tell us which countries you file in and we will send you the dates that apply to you.