My understanding of the claim is that the mainframe data is stored in a high fidelity format but some records simply lack a full year. At the time social security was instituted it was common for people to not know their own age precisely. Beyond memory issues arising from old age, most people just didn't have a pressing reason to track it. So social security benefits were granted to people who seemed old enough even though they didn't have a birth certificate or similar. The claim, as I've heard it, is that the DOGE dolts transferred this data to a more modern system where they blindly passed incomplete or placeholder values into an 8601 library implementation that uses the 2004 standard's reference date of 1875 by default.