What if the seller has to move into a nursing home before the sale, or has resided in and out of health care facilities during the 5 years before the sale? 

Section 121(d)(7) has special provisions about that. If the individual “becomes physically or mentally incapable of self-care” and resided in the home for periods of time that, in aggregate, equal at least 1 year out of the past 5, “then the taxpayer shall be treated as using such property as the taxpayer’s principal residence during any time during such 5-year period in which the taxpayer owns the property and resides in any facility (including a nursing home) licensed by a State or political subdivision to care for an individual in the taxpayer’s condition.”