To meet the Substantial Presence Test, you must be physically present in the United States on at least: 31 days during the current tax year you are asking about, and 183 days during the 3-year period that includes the year you are asking about, and the two prior years, counting:

• All the days you were present in the year you are asking about, and
• 1/3 of the days you were present in the year prior, and
• 1/6 of the days you were present in the year two years prior.

The term United States includes the following areas:

• All 50 states and the District of Columbia.
• The territorial waters of the United States.
• The seabed and subsoil of those submarine areas that are adjacent to U.S. territorial waters and over which the United States has exclusive rights under international law to explore and exploit natural resources.

You are treated as present in the United States on any day you are physically present in the country at any time during the day. However, there are exceptions to this rule for Canadian or Mexican residents that commute to the U.S. for work or foreign students, teachers and researchers, etc.

This information is found in Publication 519, U.S. Tax Guide for Aliens.