The alledged onset date, or AOD, is the date that you claim ("allege") on your Social Security application, that your disability—that is, your inability to work—began.
With Social Security Disability Insurance (SSDI), you can get retroactive pay as far back as 12 months from the date you apply for benefits—if you were disabled before that point. To get a full 12 months in backpay, you'd have to have become disabled at least 17 months before the date you applied, because there is a five-month waiting period after becoming disabled during which benefits are not paid or owed. (There is no retroactive pay for SSI—if you get approved, you'll get paid benefits from the month you apply.)
Your disability onset date determines how much in past due benefits, or backpay, you can get. For example, say that, when you applied for SSDI on 12/1/2014, you alleged that your disability began on 9/1/2014. Further suppose that the Social Security Administration (SSA) approves your benefits on 12/1/2015. If the SSA agreed with your onset date of 9/1/2014, you would be paid backpay benefits for 2/1/2015 (five months after your AOD of 9/1/2014) to 12/1/2015 (your approval date).
If, on the other hand, the SSA disagreed with your onset date and says that you weren't disabled until 2/1/2015, you would get benefits paid only from 7/1/2016 (five months after your onset date of 2/1/2015). This could mean the difference of several thousand dollars in backpay.
The onset date can also play a role in whether or not your claim is approved, since you must be disabled for 12 months (or are expected to be disabled for 12 months) to qualify for disability benefits. The date of onset is when the clock starts ticking for this 12-month durational requirement.
If the SSA disagrees with the date you say you became disabled, it can establish an onset date later than you think is correct. If the SSA sets the onset date, it's called the established onset date (EOD), rather than the alleged onset date (AOD). However, the SSA has to have contrary medical evidence to show that your alleged date is wrong and that its EOD is correct.
To determine your EOD, the SSA will look at your AOD, when you last worked, and what the medical evidence shows.
If the SSA changes your AOD to a later EOD, causing you to lose some backpay, you can appeal the new established onset date by asking Disability Determination Services (DDS) to do a reconsideration of the EOD. Or, if you already at the hearing stage, you would ask an administrative law judge or the Appeals Council to review the EOD. But beware: when you appeal the onset date, the DDS or SSA can review the disability determination and could potentially reverse your approval. If you've been approved for benefits, you should speak to a disability lawyer for help appealing an EOD.
Sometimes when Social Security changes your AOD to a later EOD, it is still more than 17 months earlier than the date you applied for disability benefits. In this case, the change in date won't matter, since you can't get paid backpay benefits for more than 12 months before your application date (plus the five-month waiting period).
If you are approved for benefits and Social Security or DDS (either an administrative law judge or a disability claims examiner) decides that your disability began when you alleged, the AOD effectively becomes the EOD.
Learn more about how your EOD affects disability back payments.