Tax
Substantial presence test
The substantial presence test is the IRS calculation determining whether a non-citizen is a US tax resident based on days physically present in the United States. It counts all days in the current year, one third of the prior year's days and one sixth of the year before that.
In plain terms: The day-count formula that decides whether the US treats you as a resident taxpayer.
Why it matters
Directly relevant to founders who travel to the US frequently for conferences, customers or fundraising. Crossing the threshold makes you a US tax resident on worldwide income, which is a materially different position from owning a US company as a non-resident.
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Source: IRS — Substantial Presence Test. This is a definition, not tax or legal advice — verify against the primary source before acting.