20 CFR 404.321: When a period of disability begins and ends.

(a) When a period of disability begins. Your period of disability begins on the day your disability begins if you are insured for disability on that day. If you are not insured for disability on that day, your period of disability will begin on the first day of the first calendar quarter after your disability began in which you become insured for disability. Your period of disability may not begin after you have attained full retirement age as defined in § 404.409.

(b) When disability ended before December 1, 1980. Your period of disability ends on the last day of the month before the month in which you become 65 years old or, if earlier, the last day of the second month following the month in which your disability ended.

(c) When disability ends after November 1980. Your period of disability ends with the close of whichever of the following is the earliest—

(1) The month before the month in which you attain full retirement age as defined in § 404.409.

(2) The month immediately preceding your termination month (§ 404.325); or

(3) If you perform substantial gainful activity during the reentitlement period described in § 404.1592a, the last month for which you received benefits.