20 C.F.R. § 404.250
Special computation rules for people who had a period of disability
If you were disabled at some time in your life, received disability insurance benefits, and those benefits were terminated because you recovered from your disability or because you engaged in substantial gainful activity, special rules apply in computing your primary insurance amount when you become eligible after 1978 for old-age insurance benefits or if you become re-entitled to disability insurance benefits or die. (For purposes of §§ 404.250 through 404.252, we use the term second entitlement to refer to this situation.) There are two sets of rules:
(a) Second entitlement within 12 months. If 12 months or fewer pass between the last month for which you received a disability insurance benefit and your second entitlement, see the rules in § 404.251; and
(b) Second entitlement after more than 12 months. If more than 12 months pass between the last month for which you received a disability insurance benefit and your second entitlement, see the rules in § 404.252.