20 CFR § 404.320 - Who is entitled to a period of disability.
(a) General. A period of disability is a continuous period of time during which you are disabled. If you become disabled, you may apply to have our records show how long your disability lasts. You may do this even if you do not qualify for disability benefits. If we establish a period of disability for you, the months in that period of time will not be counted in figuring your average earnings. If benefits payable on your earnings record would be denied or reduced because of a period of disability, the period of disability will not be taken into consideration.
(3) You file an application while disabled, or no later than 12 months after the month in which your period of disability ended. If you were unable to apply within the 12-month period after your period of disability ended because of a physical or mental condition as described in § 404.322, you may apply not more than 36 months after the month your disability ended.
(4) At least 5 consecutive months go by from the month in which your period of disability begins and before the month in which it would end.