One program I tested went from (31,12,99) to (01,01,100). Its front end formatted the date and added the century, so it showed 1 January 2000 as 01/01/19100
That wasn’t fixed. The fault didn’t affect processing (the years were wrong but had the correct offset between them) and was only visible to internal users, and also that system was expected to be retired in 2004
When it happens the repair is replacement of the screen. This is one of many reasons to check the repaiability of a device before deciding which to spend your money on
https://www.ifixit.com/repairability