How does less then 90 days in the US work? After declaring residency, can you just travel the world and not live in either?
Doing that in your first year of residency is probably not viable. You might be able to pull this off in one year in your 3 year/549 day period, assuming that you’ve already established an obvious tax home and closer connections test in years prior.
Note that you may be able to pass your presence test this way, but it will make it difficult to pass the closer connections and tax home test. You must also still spend more days in PR than in the US
Also note that PR and the IRS both have separate requirements for residency, both of which need to be met to qualify for act 60
I do not believe that you must spend more days in PR than in the US to satisfy presence if you are relying on the “less than 90 days in the US” possibility of doing so. The IRC states:
“(iii) Was present in the United States for no more than 90 days during the taxable year;”
and that is the extent of it. The IRC is the binding code if there were any conflict or ambiguity between it and another document (e.g. IRS publication 570, which in this case does not disagree: " 1. You were present in the United States for no more than 90 days during the tax year.").
There may be other good reasons to not spend too small a time in PR (e.g. tax home test and closer connections).
There is a “more days in PR than in the US” stipulation that is a part of the <$3k of earned income way of satisfying presence.