Assuming that one meets the Substantial Presence Test and therefore is deemed a Resident for tax purposes at what point in a tax year does the residency start?
For example, I spent some time in the US on vacation at the beginning of 2020 then went home to Canada. Later in 2020 I started working in the US. Together with days spent in the US in 2019 and 2018 I am well over the 183 day threshold for the Substantial Presence test.
Does my residency start at the beginning or when the threshold is met which is later in the year?
Your residency starts on the day you meet the substantial presence test. The IRS says, “If you meet the substantial presence test for a calendar year, your residency starting date is generally the first day you are present in the United States during that calendar year.”
If you are a U.S. resident for the calendar year, but you were not a U.S. resident at any time during the preceding calendar year, you are a U.S. resident only for the part of the calendar year that begins on the residency starting date.
See First Year of Residency in Publication 519 U.S. Tax Guide for Aliens
Your residency starts on the day you meet the substantial presence test. The IRS says, “If you meet the substantial presence test for a calendar year, your residency starting date is generally the first day you are present in the United States during that calendar year.”
If you are a U.S. resident for the calendar year, but you were not a U.S. resident at any time during the preceding calendar year, you are a U.S. resident only for the part of the calendar year that begins on the residency starting date.
See First Year of Residency in Publication 519 U.S. Tax Guide for Aliens