Where you owe tax is decided less by citizenship or intention than by where you physically were — and most countries write that down as a day count. This is the orientation: the vocabulary, the tests, and why the calendar is the whole game.
A tax resident is someone a jurisdiction can tax as one of its own — usually on worldwide income, not just income earned there. A non-resident is taxed only on local-source income. Same person, same year, very different bill, and the switch between the two states is usually thrown by days of physical presence.
Residency is decided country by country (and in the US, state by state on top of the federal answer). You can be resident in two places at once — that is exactly the situation day counting exists to detect, and the situation tax treaties exist to untie.
Cross a number of days of presence and you are in. The UK's Statutory Residence Test makes 183 days in the tax year automatically resident. Canada deems you resident after 183 days of sojourning. India's basic test is 182 days in the financial year. Simple to apply, unforgiving to cross by accident —run your days through the 183-day calculator.
The US federal substantial presence test counts the current year at full weight, the prior year at one-third, and the year before at one-sixth — so past presence haunts you for two more years. Both a 31-day current-year minimum and a 183-day weighted total must hold —try the SPT calculator.
Most US states and many countries layer subjective tests on top: a permanent home available, family, driver's licence, club memberships, where you vote. New York's auditors famously pair a 183-day count with domicile and then demand your day-by-day location proof. The count is the trigger; the ties decide the fight.
Every one of these tests shares one property: the burden of proof lands on you. Auditors ask where you were on a specific Tuesday eighteen months ago, and "I think I was in Portugal" loses to a passport stamp, a phone-location trail, or a credit-card receipt. People who track their days as they live them sleep; people who reconstruct them from email timestamps in audit season do not.
That is the whole idea behind Day31: the record builds itself from where you actually were, per country, per US state, per Canadian province, and every limit — Schengen's 90/180, the SPT's three-year weighting, each state's 183 — is watched before you cross it, not after.
This guide is orientation, not tax advice — residency determinations turn on statutes, treaties, and individual facts.
Day31 tracks every jurisdiction's day count automatically and warns you before a threshold. Private, offline, on your device.
