Just looked it up. 3 years, 3 months and 15 days from the end of the calendar year (April 15th of the fourth year). After that it is lost.
In your client's case, only 2019 and later will be added to their SS record.
I learned something new today, I assumed you would get that credit as long as you filed the returns before you filed for SS benefits. Thanks for the post.
Tom
Longview, TX