What does it mean if an applicant receives a letter that says s/he has been found "not disabled as of a certain date"?

This letter applies to an individual’s eligibility for Social Security Disability Insurance (SSDI). SSDI is an insurance program that you pay into when you work through a portion of FICA payroll taxes. As such, when you stop working, you stop paying into this program and eventually your “insured” status will expire. The date your insured status for SSDI expires is known as the “date last insured.” For SSDI, applicants need to show that they were disabled before the date last insured in order to be eligible for benefits. For the letter in this question, it was found that the applicant was not disabled before his/her date last insured. If the applicant decides to appeal the decision, locating medical records the document an earlier onset of disability (if applicable) and checking the applicant’s earnings record to see if any earnings were missing are key strategies to pursue.

If the applicant’s disability arose after the date last insured, s/he may still be eligible for SSI, if h/she meets SSI resource and income requirements.