When exactly does my residency starting date fall in the year I pass the substantial presence test?

Under the IRS rules, your residency starting date is generally the first day you were physically present in the U.S. during the calendar year you pass the substantial presence test. Days before that date make you a nonresident, and days on and after it make you a resident, creating a dual-status year. There's a de minimis rule that lets you ignore up to 10 days of presence connected to a closer foreign connection. If you qualify under the green-card test instead, your start date is the first day you're a lawful permanent resident. Getting this date right matters because it decides when your worldwide income becomes U.S.-taxable. See IRS Publication 519.