How exactly do I count days for the substantial presence test with the 1/3 and 1/6 weighting?

The IRS substantial presence test adds up days over three years. You count all days in the current year, one-third of your days in the prior year, and one-sixth of your days in the year before that. If the weighted total reaches 183 and you were physically present at least 31 days in the current year, you're treated as a resident alien. For example, 120 days in each of three years gives 120 + 40 + 20 = 180, which is under 183, so you'd stay a nonresident. Partial days generally count as full days. Keep a travel log with entry and exit dates, since the IRS puts the burden on you. The IRS explains this at irs.gov under 'Substantial Presence Test.'