DLI stands for Date Last Insured. It can be one of the most important factors in a claim for Social Security Disability Insurance Benefits ("DIB"). This entry addresses the non-medical requirements for benefits. A person qualifies for DIB by working and paying into the Social Security system. Each year is broken up into quarters (Jan - March, April - June, etc.) and eligibility is based on the total number of quarters earned AND the time frame in which they are earned. For more information of how to earn credits and how many quarters are required for coverage, see here.
Once you stop working, of course, you stop paying into the Social Security program. At that point, you stop earning credits toward your eligibility, and your eligibility will eventually end. Your coverage may extend out into the future up to, generally speaking, a maximum of 5 years. This is where your date last insured comes in. Based on your earnings history, Social Security will determine the date in which your eligibility to receive DIB ends. This is called your date last insured, and in order to receive disability benefits, you must prove that you became disabled on or before this date.
When you apply for Social Security disability benefits, Social Security will first determine (1) whether or not you have worked and earned enough credits to receive DIB and (2) what your date last insured is. If you have not worked long enough to earn credits, Social Security will send you a letter telling you that you cannot receive Disability Insurance Benefits because you have not worked long enough; they will not make any medical findings in your case. If you have worked long enough, Social Security will determine your DLI and then make a medical determination deciding whether you were disabled on or before that date.