When explaining all the many concepts that go into Social Security Disability, one that often confuses people is the idea of a Date Last Insured or "DLI". The long details can be found here on the Social Security website and I encourage you to read more about it for your individual Disability case, but below I will try to briefly simplify it.
Determining whether your case is SSDI, SSI, or both would be the first step in figuring out if Date Last Insured applies to your claim; this is because Date Last Insured only applies to SSDI cases. This is because Date Last Insured is calculated by the last day you worked a job that pays FICA taxes. Social Security often uses what's called the 20/40 test which evaluates whether you have worked 20 of the last 40 quarters earning over substantial gainful activity, essentially whether you worked full time 5 of the last 10 years. If that is the case, you are likely to be considered fully insured by Social Security.
Ok, so what does that mean? Well if you are not considered insured by Social Security (worked 20 of the last 40 quarters full time), then you may need to be concerned about have a Date Last Insured that is in the past. This is a problem because for Social Security Disability purposes, you must prove that you were disabled by your DLI.
For example, many people I see have Date Last Insured's in the future like "June 30, 2019". So if that's your date, no problem; your case can proceed as normal. However, I have also seen individual's with DLI's of "December 31, 2013" which is a problem because then you and your Social Security Disability attorney must prove you were disabled before that date.
The farther back your date, the more difficult it is to prove. Why? Well collecting medical records from 5-10 years ago can be quite a chore. Plus, Social Security will essentially be asking you what kinds of symptoms you were having or what a typical day was like back then. That is not an easy thing to do.