“Back pay refers to the benefits a claimant is owed for the period between the date they became entitled to benefits and the date the Social Security Administration (SSA) approves the claim. Two dates drive the calculation: the established onset date, which is when the SSA determines the disability began, and the application date. For Social Security Disability Insurance (SSDI), benefits can reach back to the onset date, subject to a mandatory five-month waiting period during which no benefits are paid.”
How Back Pay and Retroactive Benefits Work in Social Security Disability Cases – USA Today