26 CFR §1.121-5

1. (a)In general. Under section 121(d)(9), a taxpayer who is serving (or whose spouse is serving) on qualified official extended duty as a member of the uniformed services or Foreign Service of the United States may elect to suspend the running of the 5-year period of ownership and use during such service but for not more than 10 years. The election does not suspend the running of the 5-year period for any period during which the running of the 5-year period with respect to any other property of the taxpayer is suspended by an election under section 121(d)(9).
2. (b)Manner of making election. The taxpayer makes the election under section 121(d)(9) and this section by filing a return for the taxable year of the sale or exchange of the taxpayer's principal residence that does not include the gain in the taxpayer's gross income.
3. (c)Application of election to closed years. A taxpayer who would otherwise qualify under §§ 1.121-1 through 1.121-4 to exclude gain from a sale or exchange of a principal residence on or after May 7, 1997, may elect to apply section 121(d)(9) and this section for any years for which a claim for refund is barred by operation of any law or rule of law by filing an amended return before November 11, 2004.
4. (d)Example. The provisions of this section are illustrated by the following example:
5. (e)Effective date. This section is applicable for sales and exchanges on or after May 7, 1997.