26 U.S. Code § 121 - Exclusion of gain from sale of principal residence

(C) Period of nonqualified use For purposes of this paragraph—

(i) In general

The term “period of nonqualified use” means any period (other than the portion of any period preceding January 1, 2009) during which the property is not used as the principal residence of the taxpayer or the taxpayer’s spouse or former spouse.

(ii) Exceptions

The term “period of nonqualified use” does not include—

(I) any portion of the 5-year period described in subsection (a) which is after the last date that such property is used as the principal residence of the taxpayer or the taxpayer's spouse,

(II) any period (not to exceed an aggregate period of 10 years) during which the taxpayer or the taxpayer's spouse is serving on qualified official extended duty (as defined in subsection (d)(9)(C)) described in clause (i) , (ii), or (iii) of subsection (d)(9)(A) , and

(III) any other period of temporary absence (not to exceed an aggregate period of 2 years) due to change of employment, health conditions, or such other unforeseen circumstances as may be
specified by the Secretary