You will have your two years in for the exclusion on
12-31-2017
, as the ownership/residence clock would have started 1-1-2016 based on your facts above.
You must have owned the home for at least two years (730 days or 24 full months) during the five years prior to the date of your sale. It doesn't have to be continuous, nor does it have to be the two years immediately preceding the sale.
For more detail see: https://turbotax.intuit.com/tax-tools/tax-tips/Home-Ownership/Tax-Aspects-of-Home-Ownership--Selling...
The IRS question is a difficult one to answer. If you have some real doubts about the 1-1-2016 being valid, then to be safe you may want to consider the May date. Without knowing all the detail I can't give you a more detailed answer.
