Two countries. One set of days. One clear record.
When two countries each call you a resident, you don't get to pick, a tax treaty does, through a tie-breaker that walks down a ladder: permanent home, center of vital interests, habitual abode, then nationality. Almost every rung turns on where you actually were. Tax Days counts your days in each jurisdiction so you can answer the tie-breaker with evidence instead of guesswork.
What you're up against
Two countries can both claim you as a resident
Citizenship, a home, family, or enough days in each place can make you a tax resident of both countries in the same year, exposing the same income to tax twice until a treaty sorts it out.
The tie-breaker is a ladder, not a coin flip
Most treaties resolve dual residency in order: permanent home available to you, then center of vital interests, then habitual abode, then nationality. You move down only when a rung is genuinely a tie, and several rungs hinge on where you spent your days.
If you're a US citizen, the saving clause can override the treaty
Under the saving clause in US treaties, the US generally reserves the right to tax its citizens as if the treaty didn't exist. Becoming a treaty resident of the other country usually does not switch off US filing or US tax on worldwide income.
"Habitual abode" comes down to your day pattern
If your permanent home and vital interests are split, the tie-breaker looks at where you stay more regularly and frequently, your habitual abode. Without a contemporaneous day log, that's an argument you can lose.
What Tax Days does for you
Counts your days in each country and region so you can see, at a glance, where you actually spent the year.
Feeds every tie-breaker rung, permanent home, center of vital interests, habitual abode, nationality, with the day pattern each step depends on.
Flags when you're on track to be a tax resident of more than one country in the same year, before the year closes.
Tracks any-part-of-a-day rules and travel days so a single misremembered trip doesn't quietly flip your status.
Keeps a contemporaneous, time-stamped log, the kind of record that holds up far better than a reconstruction after the fact.
Exports an audit-ready PDF per jurisdiction with day counts, trips, and thresholds to hand to your cross-border accountant or attorney.
Stays on your iPhone. No servers, no accounts, no logs. Optional iCloud sync.
The rules you actually need to track
Treaty tie-breaker rules
How the residency tie-breaker ladder works, permanent home, center of vital interests, habitual abode, then nationality, and where day counts decide the outcome.
Read the guide →The US saving clause
Why winning the treaty tie-breaker rarely frees a US citizen from US tax, and how the saving clause reshapes what dual residency means for you.
Read the guide →Substantial Presence Test
The day-count formula that can make a non-US national a US tax resident, current year plus weighted prior-year days, before any treaty even applies.
Read the guide →Closer Connection Exception
An escape from US residency under the SPT if you keep your tax home and closer ties abroad and stay under the day limit. A precursor to the treaty analysis.
Read the guide →SPT vs. green card test
The two paths to US resident status, how each interacts with a treaty position, and why dual citizens need to know which one applies to them.
Read the guide →183-day rules around the world
How the common 183-day threshold triggers residency in the other country, and feeds straight into the habitual abode and day-count tie-breaker steps.
Read the guide →
Free guides and tools
- Guide
Treaty tie-breaker rules, explained
Walk the tie-breaker ladder step by step and see exactly which rungs your day counts decide.
- Guide
Expat day counter guide
Why a contemporaneous day count is the backbone of any cross-border residency position.
- Calculator
Free 183-day rule calculator
Set the other country's threshold and see when your days would cross into residency there.
- Calculator
Substantial Presence Test calculator
Run the weighted US day-count formula to see if you'd be a US tax resident before treaty relief.
Common questions
Does dual citizenship mean paying tax in both countries?
Not automatically. Most countries tax by residency, not citizenship, so where you actually live and spend days matters more than the passports you hold. The big exception is the US, which taxes its citizens on worldwide income wherever they live.
What happens if both countries claim me as a tax resident?
If a tax treaty exists, its tie-breaker rules usually assign you to one country for treaty purposes, looking at your permanent home, center of vital interests, habitual abode, and nationality in that order. Each step leans heavily on where you actually spent your time.
Can I choose which country I'm a tax resident of?
You can influence it, not declare it. Residency follows facts: days present, homes maintained, where family and economic life sit. Aligning those facts deliberately, and documenting them, is what actually moves the answer.
Does a treaty tie-breaker help a US citizen?
Less than most people hope. US treaties contain a saving clause that generally preserves the US right to tax its own citizens, so the tie-breaker mainly shapes what the other country can tax and which credits apply. Specifics vary by treaty, so get advice on your pair of countries.
Settle the tie-breaker with evidence, not estimates
When two countries claim you, the days decide. Tax Days keeps the record your cross-border accountant needs for £19.99/year, a fraction of one hour of professional time.
