When the Social Security field rep or online disability application asks you when you became unable to work, it's asking for your "alleged onset date" of disability. That means the date you're claiming your disability began, which should generally be the last time you were able to do any significant amount of work. (In 2021, Social Security considers a significant amount of work to be $1,310 per month or more—what it calls the substantial gainful activity level.) If you work after the onset date you claim on your application, this can cause problems for your disability case, so you need to choose the date you became unable to work carefully.
If you made an attempt to return to work, but stopped, then you may be able to legitimately use the date you originally stopped working as your disability onset date. To do this, you have to get Social Security to ignore the work you did recently. Whether you can do this and use the date of the second time you stopped working depends on the facts of your situation, including how much you earned while you were working the second time, and how long you worked for during that second work period:
To qualify as an unsuccessful work attempt (UWA), you must have quit because your medical condition made it impossible for you to do the work, or because your doctor restricted you from doing some of the tasks required, or because the employer took away special accommodations, such as special equipment or permission to work a flexible schedule, that were making it possible for you to work, or other special conditions were removed (for instance, someone else got you ready for work and took you to and from work, and that person was no longer able to help).
If your work period qualifies as an unsuccessful work attempt, you can use the date you originally stopped working as your alleged onset date. But unless you're sure Social Security will count the work as an unsuccessful work attempt, it's usually best to choose an onset date that's after the last day you did any significant amount of work. That's because, if Social Security disagrees with your alleged onset date, you'll likely have to go to an appeal hearing to get the onset date you want. And even then, the administrative law judge may try to move up your onset date so that you aren't paid back payments for any period you worked. For more information, see our article on what to consider when choosing the date you became unable to work.
Updated January 6, 2021