When you apply for disability benefits, you allege an “onset date,” i.e. the date on which you believe you became disabled. This is called the Alleged Onset Date (AOD). Most typically, the AOD is the date that you stopped working due to your impairments. The Social Security Administration (SSA) can find that you became disabled prior to that date, on that date, or after that date, depending on numerous factors, including your work activity (maybe you worked too much after the alleged onset date but stopped later), medical evidence (maybe medical evidence showed very serious change in your condition for the worse before after that date), age (maybe you attained age 50 or 55, resulting in a different application of the medical-vocational guidelines), etc. The date on which SSA finds you disabled is called the Established Onset Date (EOD). The EOD is what controls the amount of retroactive benefits (back pay).
The amount of retroactive benefits payable on a disability claim depends on the type of claim (SSD vs. SSI), the application date (more specifically, the protective filing date), and the five-month waiting period. SSA will pay retroactive benefits on SSD claims up to a maximum of twelve months prior to the application date. For example, if you applied in June 2024 and SSA found you disabled back to January 2021, SSA will pay you retroactive benefits only back to June 2023. On SSI claims, SSA can only pay from the application date forward. For example, if you applied for SSI in June 2024 and SSA makes a favorable determination in December 2024, SSA can only pay the SSI retroactive benefits from June 2024 forward.
Lastly, for SSD, a “five month waiting period” is written into the program, and this five month waiting period states that SSA simply does not pay benefits for the first five months after the established onset date month. This means you receive your first payment in the sixth full month after the date that your disability began. For example, if SSA finds you disabled as of March 15, 2024, you do not receive a payment for the onset month (March 2024) and you do not receive a payment for the next five months (April through August 2024). Your first payment would be for September 2024. SSA sends payment in the month following the month for which they are due, so the check for October 2024 would be your first check received.
SSA gives some more examples of when your benefits could start on its website.