I recently moved from UK to US and I am having trouble understanding tax residency matters. In particular, it seems to me that I am not resident in any of the two countries. Indeed:
I pass the first overseas test for UK tax residency: "You were resident in the UK for 1 or more of the 3 tax years preceding the tax year, and you spend fewer than 16 days in the UK in the tax year". Tax year in UK ended on April 5th 2018 and at that time I had already moved to the US. So I spent 0 days in the UK in this UK tax year. So I am not UK tax resident as of today.
In the US I am nonresident alien since I don't have green card and I don't pass the substantial presence test (I have been in the US for less than 183 days, as of today). In my understanding, being nonresident alien implies being non resident for tax purposes. So I am not a US tax resident as of today.
Is this analysis correct? It seems strange to me not to be resident for tax purposes in any country.
On a more general note, is tax residency something related to a specific day (for example "as of today, I am tax resident in such and such countries") or to a whole tax year? If I spend more than 16 days in the UK next October, do I become retroactively UK tax resident as of today?
Edit: I am on O-1 visa