20 CFR 404.120: How we determine currently insured status.

(a) What the period is for determining currently insured status. You are currently insured if you have at least 6 quarters of coverage (QCs) during the 13-quarter period ending with the quarter in which you—

(1) Die;

(2) Most recently became entitled to disability insurance benefits; or

(3) Became entitled to old-age insurance benefits.

(b) What quarters are not counted as part of the 13-quarter period. We do not count as part of the 13-quarter period any quarter all or part of which is included in a period of disability established for you, except that the first and last quarters of the period of disability may be counted if they are QCs (see § 404.146(d)).