There are two different indexes based on Social Security information.  

One is the SSDI or Social Security Death Index which lists the name, date of death, and formerly the SSN, and some other info.  It used to be a separate index, but now is listed as part of the database of Ancestry and some other online sites.

The other is the SSACI, the Social Security Applications and Claims index which is what you are referencing.  It lists information on the application and claims against the account. So the date listed there would reflect when a claim was initiated. In my experience the amount of data found on SSACI listings varies considerably from next to nothing to  birth date, name changes, death date, and a variety of other useful information.  I don't know why there is such a disparity in the amount of information from one person to the next.  I have seen multiple SSACI listings for the same person, with different data, or amounts of data.

A fairly good explanation of this can be found on the Legacy Tree website at https://www.legacytree.com/blog/using-social-security-administration-records-genealogy

Rich Meyersburg
Laurel, MD