You get an amended onset date when, after applying for disability benefits, Social Security changes the date that you claimed your disability began. To explain how and why this happens, we first need to define some terms.
A fully favorable decision is where an SSA administrative law judge (ALJ) grants an applicant disability benefits back to the alleged onset date. A partially favorable decision is when the ALJ estabishes an onset date, or date of disability, that is less favorable than the original AOD (that is, it's more recent). (It's less favorable because the applicant receives less in retroactive back payments, due to the later date of disability. Read more about fully vs. partially favorable decisions.)
Here is where the "amended onset date" comes in. It happens quite frequently that an administrative law judge will decide to approve a claim with an onset date that is less favorable than the date that was alleged on the application and yet still call it a fully favorable decision. How is this possible? It happens in cases where a judge convinces a claimant to agree to "an amended onset date" in exchange for receiving a fully favorable decision. That is, the claimant agrees to change their statement regarding when their disability began.
Why does the judge want to be able to call the decision fully favorable? By convincing the claimant to amend their onset date and then calling the decision "fully favorable," it's highly unlikely that, if the case is appealed, the Appeals Council would overturn or send the case back to the ALJ.
By: Tim Moore, former Social Security claims examiner