If you are a non-US person who happens to spend a lot of time in the United States you are secure in the fact that you will not become liable for US taxes on your worldwide income so long as you do not spend 183 days or more in the United States during any tax year; RIGHT?
WRONG! To avoid a very dangerous pitfall you must be careful to read all of the fine print in the rules that determine US tax residency for non-US persons.
First, if you spend even one minute of one 24 hour day in the US, that counts as a day towards tax residency. Consequently, if you are flying through a US hub airport and land on US soil at 11:59 p.m. and then quickly change planes and leave again at 12:45 a.m., you will still have spent two tax days in the United States; even though one of them was only one minute long, and the other was just 45 minutes long.
Second, you have to learn how to count all over again. Under the law, the way the number of tax residency days is determined is to count: all of the days present in the US during the current tax year; plus, one third of the days present in the US during the previous tax year; plus, one sixth of the days present in the US during the year before that.
If you spend a lot of time in the US, this will eventually mean, that in any given year, you will be limited to about 121 days--anything beyond that, when calculated as above could make you tax resident.
Many unsuspecting people have been caught in this trap, and end up negotiating a tax payment to the IRS that was entirely unplanned and unexpected.
Don't let this happen to you!
