Many Canadian snowbirds plan their winters around one number: 183 days. Stay under it, the thinking goes, and the IRS has no claim on you.

That’s not how the test works.

The formula counts three years, not one

The US Substantial Presence Test doesn’t just look at the current year. It’s a weighted three-year formula: all of your US days this year, plus one-third of last year’s days, plus one-sixth of the days from the year before. If you were in the US at least 31 days this year and the weighted total reaches 183, you meet the test — and meeting the test means the US considers you a resident for tax purposes, taxable on worldwide income.

Run the numbers on a typical snowbird pattern — roughly four months in the US every winter, year after year — and the weighted total crosses the line even though no single year comes close to 183 actual days. A consistent 120-day winter can be enough. The day you arrive and the day you leave each count as full days. Partial days count.

There are narrow carve-outs: a medical condition that arose during the US stay, and transit days under 24 hours between two non-US countries. They rarely help a snowbird who simply likes long winters.

The exception most snowbirds rely on

Meeting the test is not the end of the story. A snowbird who was in the US fewer than 183 actual days in the current year, and whose home — economically, socially, personally — is genuinely in Canada, can claim the closer connection exception by filing Form 8840 with the IRS.

This exception is for non-U.S. citizens and non-green-card holders. U.S. citizens and green-card holders are already in the U.S. tax system under different rules.

The form is short. It asks where your permanent home is, where you bank, where you vote, where your family and belongings are. Filed on time, and assuming the factual ties support it, it can preserve nonresident alien status.

The catch is the word filed. The exception isn’t automatic. A snowbird who meets the Substantial Presence Test and never files Form 8840 hasn’t claimed anything — and if the question comes up later, they’re left arguing residency under the The US–Canada Tax Treaty: What It Protects You From — and What It Doesn’t tie-breaker rules instead, which means more filings, not fewer. And a snowbird who spends 183 or more actual days in the current year can’t use the closer connection exception at all.

The quiet part

Nothing announces that you’ve met the test. No letter arrives. Border-crossing records are increasingly accessible and should be assumed reconstructable, so the day counts exist whether or not anyone has added them up yet.

The failure mode here isn’t dramatic. It’s a retired couple who have wintered in Arizona for six years, never counted a partial day, and never heard of Form 8840 — whose situation is fine right up until it’s examined.

If your winters follow a pattern, the three-year math is worth running once. It takes ten minutes, and it tells you whether a two-page form should be part of your annual routine.

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