I arrived on the 30th of December 2015, in order to start my work (Research scholar) in early January 2016. Which means the system consider 2015 as my first calendar year in the US, even if I was there for only 2 days AND unemployed, which is completely stupid but whatever.
I left the US on the 28th of December 2017. My tax treaty was from the 30th of December 2015 to the 28th of December 2017. Which means for 2017 I was technically a US resident (thanks to this calendar year rule), but still under tax treaty, so tax-exempted, so non-resident for US tax purpose.
