THis is on the instructions for Form 1040 NR-EZ (www.irs.gov/pub/irs-pdf/i1040nre.pdf)

Substantial Present Test.

You will be considered a U.S. resident for tax purposes if you meet the substantial presence test for calendar year 2008. To meet this test, you must be physically present in the United States on at least:

1.) 31 days during 2008,

and

2.) 183 days during the 3-year period that includes 2008, 2007, and 2006, counting

All the days you were present in 2008, and

â…“ of the days you were present in 2007, and

â…™ of the days you were present in 2006