The substantial presence test is the United States rule that determines whether a non-citizen is treated as a US tax resident based on days of physical presence. A person generally meets the test if present in the United States at least 31 days in the current year and 183 days over a three-year period, counting all days in the current year, one-third of the days in the prior year, and one-sixth of the days in the year before that.
Meeting the test can make a person subject to United States tax on worldwide income. The rule is significant for Canadian snowbirds who spend extended winters in the United States. Relief may be available through the closer-connection exception, claimed on Form 8840, or through the residency tie-breaker in the Canada-United States tax treaty, each of which requires its own conditions and filings to be met.