Generally, there's two dates to consider here, and the IRS considers the date of conversion to to the transition date. For example, I moved out of my house on Jan 18, 2004. Jan 19, 2004 was the date I converted it to a rental, and began working on it to get it ready. Jan 24 was my "in service" date, because that's the earliest date a renter "could" have moved in. Therefore, the last day it was my primary residence was Jan 18, 2004. My new "primary residence" day count began with day 1 on Jan 19, in the house I moved in to.
