Many people don’t realize that you can receive both Social Security as well as 401k income. Whatever income you receive from your 401k plan will also not affect your Social Security benefits, since it is considered income from a non-wage source.
These are two terms that you will hear often, and the main difference comes down to the central focus of work history. They are important to differentiate since they are two entirely different government programs.
The short answer is that your disability onset date is the date on which your disability first prevented you from working. Your disability onset date will be important when working with the Social Security Administration.