Just a clarification, start with SSI before calling your Credit Union. Social Security checks directly deposited into your account are ACH (Automated Clearing House) transactions. Essentially, they are electronic payments, not physical checks. Once they show up on the Credit Union's ACH list, they are diverted into the correct account that day. Financial Institutions are required to make those funds available the day they show up. My point is that the Credit Union will have no further information than you already do as to why the date was changed.