You moved states — do you still owe tax to the old one?

Domicile is your one true home: the place you intend to return to. It does not change because you signed a lease elsewhere. It changes when the centre of your life moves — home, family, work, belongings, the things you actually do. Until it does, the old state can tax your worldwide income however few days you spend there.

Statutory residency is the day count, and it applies even if your domicile has successfully moved. New York's version is the strictest of the common ones: more than 183 days in the state, plus a permanent place of abode maintained there, and you are taxed as a resident regardless of where you are domiciled. Keeping an apartment "for visits" is the classic way to fail a move you otherwise made properly.

California works differently again — no fixed day line, but a nine-month presumption of residency and a closest-connections test underneath it.

Because the two tests are independent, the answer "I only spent a few months there" settles one of them and says nothing about the other.

State residency audits are not a paper exercise about your driving licence. New York's own nonresident audit guidelines set out the approach, and the evidence is granular: where you were on each day, and what your life looked like around those days.

In practice that means day-level records — and the records that corroborate them, which you do not control: card transactions, phone records, toll and travel data, building entry logs. Auditors reconcile what you claim against what those show.

The asymmetry is the point. The state's evidence is generated automatically; yours is not, unless you kept it. A reconstructed calendar produced two years later, from memory, against a record that was made contemporaneously, is a weak position however true it is.

Statutory residency tests generally have no gradient. In New York, 183 days makes you a resident and 182 does not — the tax consequence of one day is the whole year's worldwide income at resident rates.

Two things follow. Any part of a day generally counts, so a morning flight in and an evening flight out are two days, not one. And the year to watch is the year you move, when both states have a claim and the count is at its most contested.

Changing domicile, allocating income across two states, and defending a filing position are fact-specific and expensive to get wrong. What is worth doing before any of that is the part nobody else can do for you: keep a dated record of where you were, from the day you decide to move rather than from the day an auditor asks.

• New York State Department of Taxation and Finance — Income tax definitions (residency)
• New York State Department of Taxation and Finance — Nonresident audit guidelines
• California Franchise Tax Board — Publication 1031, Guidelines for Determining Resident Status