I've been investigating this topic to ensure I don't get caught by any surprises during our trip planned for next year.
A Substantial Presence Test is used to determine if you are considered to be a US resident and it's based on the number of days in the US over the last 3 years using a formula. If you are considered a US resident then you will have to file a US Tax return.
My interpretation is that it's possible to be in the US more than 183 days over 2 calendar years (ie. fall 2015 - spring 2016). For example if you were in the US for 240 days (120 days in 2015 & 120 days in 2016) with no other past US visits then:
- For 2015: You would not be considered a US resident since the number of days is 120 days which is < 183.
- For 2016: The calculation is the current year 2016 (120 days) + 1/3 of previous year 2015 (120 / 3 = 40) + 1/6 of 2014 (0) which = 160 days. Thus you would not be considered a US resident since 160 < 183.
I'm definitely not an expert on this subject. I'm just trying to educate myself to ensure I don't get caught by any surprises. I suggest that any interested parties refer to a document published by BDO in March 2015 that explains US Tax issues for Canadians:
http://www.bdo.ca/en/Library/Services/Tax/Documents/Tax-Bulletins/US-Tax-Issues-for-Canadians.pdf